diff --git a/Documentation/config.txt b/Documentation/config.txt
index da74ad0d0a..1a8a399dd3 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -769,6 +769,14 @@ am.keepcr::
 	by giving '--no-keep-cr' from the command line.
 	See linkgit:git-am[1], linkgit:git-mailsplit[1].
 
+am.threeWay::
+	By default, `git am` will fail if the patch does not apply cleanly. When
+	set to true, this setting tells `git am` to fall back on 3-way merge if
+	the patch records the identity of blobs it is supposed to apply to and
+	we have those blobs available locally (equivalent to giving the `--3way`
+	option from the command line). Defaults to `false`.
+	See linkgit:git-am[1].
+
 apply.ignoreWhitespace::
 	When set to 'change', tells 'git apply' to ignore changes in
 	whitespace, in the same way as the '--ignore-space-change'
diff --git a/Documentation/git-am.txt b/Documentation/git-am.txt
index 0d8ba48f79..dbea6e7ae9 100644
--- a/Documentation/git-am.txt
+++ b/Documentation/git-am.txt
@@ -10,7 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git am' [--signoff] [--keep] [--[no-]keep-cr] [--[no-]utf8]
-	 [--3way] [--interactive] [--committer-date-is-author-date]
+	 [--[no-]3way] [--interactive] [--committer-date-is-author-date]
 	 [--ignore-date] [--ignore-space-change | --ignore-whitespace]
 	 [--whitespace=<option>] [-C<n>] [-p<n>] [--directory=<dir>]
 	 [--exclude=<path>] [--include=<path>] [--reject] [-q | --quiet]
@@ -90,10 +90,13 @@ default.   You can use `--no-utf8` to override this.
 
 -3::
 --3way::
+--no-3way::
 	When the patch does not apply cleanly, fall back on
 	3-way merge if the patch records the identity of blobs
 	it is supposed to apply to and we have those blobs
-	available locally.
+	available locally. `--no-3way` can be used to override
+	am.threeWay configuration variable. For more information,
+	see am.threeWay in linkgit:git-config[1].
 
 --ignore-space-change::
 --ignore-whitespace::
diff --git a/Makefile b/Makefile
index 7efedbe834..e39ca6ca64 100644
--- a/Makefile
+++ b/Makefile
@@ -467,7 +467,6 @@ TEST_PROGRAMS_NEED_X =
 # interactive shell sessions without exporting it.
 unexport CDPATH
 
-SCRIPT_SH += git-am.sh
 SCRIPT_SH += git-bisect.sh
 SCRIPT_SH += git-difftool--helper.sh
 SCRIPT_SH += git-filter-branch.sh
@@ -813,6 +812,7 @@ LIB_OBJS += xdiff-interface.o
 LIB_OBJS += zlib.o
 
 BUILTIN_OBJS += builtin/add.o
+BUILTIN_OBJS += builtin/am.o
 BUILTIN_OBJS += builtin/annotate.o
 BUILTIN_OBJS += builtin/apply.o
 BUILTIN_OBJS += builtin/archive.o
diff --git a/builtin.h b/builtin.h
index 839483de6e..79aaf0afe8 100644
--- a/builtin.h
+++ b/builtin.h
@@ -30,6 +30,7 @@ extern int textconv_object(const char *path, unsigned mode, const unsigned char
 extern int is_builtin(const char *s);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);
+extern int cmd_am(int argc, const char **argv, const char *prefix);
 extern int cmd_annotate(int argc, const char **argv, const char *prefix);
 extern int cmd_apply(int argc, const char **argv, const char *prefix);
 extern int cmd_archive(int argc, const char **argv, const char *prefix);
diff --git a/builtin/am.c b/builtin/am.c
new file mode 100644
index 0000000000..1399c8dd88
--- /dev/null
+++ b/builtin/am.c
@@ -0,0 +1,2321 @@
+/*
+ * Builtin "git am"
+ *
+ * Based on git-am.sh by Junio C Hamano.
+ */
+#include "cache.h"
+#include "builtin.h"
+#include "exec_cmd.h"
+#include "parse-options.h"
+#include "dir.h"
+#include "run-command.h"
+#include "quote.h"
+#include "lockfile.h"
+#include "cache-tree.h"
+#include "refs.h"
+#include "commit.h"
+#include "diff.h"
+#include "diffcore.h"
+#include "unpack-trees.h"
+#include "branch.h"
+#include "sequencer.h"
+#include "revision.h"
+#include "merge-recursive.h"
+#include "revision.h"
+#include "log-tree.h"
+#include "notes-utils.h"
+#include "rerere.h"
+#include "prompt.h"
+
+/**
+ * Returns 1 if the file is empty or does not exist, 0 otherwise.
+ */
+static int is_empty_file(const char *filename)
+{
+	struct stat st;
+
+	if (stat(filename, &st) < 0) {
+		if (errno == ENOENT)
+			return 1;
+		die_errno(_("could not stat %s"), filename);
+	}
+
+	return !st.st_size;
+}
+
+/**
+ * Like strbuf_getline(), but treats both '\n' and "\r\n" as line terminators.
+ */
+static int strbuf_getline_crlf(struct strbuf *sb, FILE *fp)
+{
+	if (strbuf_getwholeline(sb, fp, '\n'))
+		return EOF;
+	if (sb->buf[sb->len - 1] == '\n') {
+		strbuf_setlen(sb, sb->len - 1);
+		if (sb->len > 0 && sb->buf[sb->len - 1] == '\r')
+			strbuf_setlen(sb, sb->len - 1);
+	}
+	return 0;
+}
+
+/**
+ * Returns the length of the first line of msg.
+ */
+static int linelen(const char *msg)
+{
+	return strchrnul(msg, '\n') - msg;
+}
+
+/**
+ * Returns true if `str` consists of only whitespace, false otherwise.
+ */
+static int str_isspace(const char *str)
+{
+	for (; *str; str++)
+		if (!isspace(*str))
+			return 0;
+
+	return 1;
+}
+
+enum patch_format {
+	PATCH_FORMAT_UNKNOWN = 0,
+	PATCH_FORMAT_MBOX,
+	PATCH_FORMAT_STGIT,
+	PATCH_FORMAT_STGIT_SERIES,
+	PATCH_FORMAT_HG
+};
+
+enum keep_type {
+	KEEP_FALSE = 0,
+	KEEP_TRUE,      /* pass -k flag to git-mailinfo */
+	KEEP_NON_PATCH  /* pass -b flag to git-mailinfo */
+};
+
+enum scissors_type {
+	SCISSORS_UNSET = -1,
+	SCISSORS_FALSE = 0,  /* pass --no-scissors to git-mailinfo */
+	SCISSORS_TRUE        /* pass --scissors to git-mailinfo */
+};
+
+struct am_state {
+	/* state directory path */
+	char *dir;
+
+	/* current and last patch numbers, 1-indexed */
+	int cur;
+	int last;
+
+	/* commit metadata and message */
+	char *author_name;
+	char *author_email;
+	char *author_date;
+	char *msg;
+	size_t msg_len;
+
+	/* when --rebasing, records the original commit the patch came from */
+	unsigned char orig_commit[GIT_SHA1_RAWSZ];
+
+	/* number of digits in patch filename */
+	int prec;
+
+	/* various operating modes and command line options */
+	int interactive;
+	int threeway;
+	int quiet;
+	int signoff;
+	int utf8;
+	int keep; /* enum keep_type */
+	int message_id;
+	int scissors; /* enum scissors_type */
+	struct argv_array git_apply_opts;
+	const char *resolvemsg;
+	int committer_date_is_author_date;
+	int ignore_date;
+	int allow_rerere_autoupdate;
+	const char *sign_commit;
+	int rebasing;
+};
+
+/**
+ * Initializes am_state with the default values. The state directory is set to
+ * dir.
+ */
+static void am_state_init(struct am_state *state, const char *dir)
+{
+	int gpgsign;
+
+	memset(state, 0, sizeof(*state));
+
+	assert(dir);
+	state->dir = xstrdup(dir);
+
+	state->prec = 4;
+
+	git_config_get_bool("am.threeway", &state->threeway);
+
+	state->utf8 = 1;
+
+	git_config_get_bool("am.messageid", &state->message_id);
+
+	state->scissors = SCISSORS_UNSET;
+
+	argv_array_init(&state->git_apply_opts);
+
+	if (!git_config_get_bool("commit.gpgsign", &gpgsign))
+		state->sign_commit = gpgsign ? "" : NULL;
+}
+
+/**
+ * Releases memory allocated by an am_state.
+ */
+static void am_state_release(struct am_state *state)
+{
+	free(state->dir);
+	free(state->author_name);
+	free(state->author_email);
+	free(state->author_date);
+	free(state->msg);
+	argv_array_clear(&state->git_apply_opts);
+}
+
+/**
+ * Returns path relative to the am_state directory.
+ */
+static inline const char *am_path(const struct am_state *state, const char *path)
+{
+	return mkpath("%s/%s", state->dir, path);
+}
+
+/**
+ * If state->quiet is false, calls fprintf(fp, fmt, ...), and appends a newline
+ * at the end.
+ */
+static void say(const struct am_state *state, FILE *fp, const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	if (!state->quiet) {
+		vfprintf(fp, fmt, ap);
+		putc('\n', fp);
+	}
+	va_end(ap);
+}
+
+/**
+ * Returns 1 if there is an am session in progress, 0 otherwise.
+ */
+static int am_in_progress(const struct am_state *state)
+{
+	struct stat st;
+
+	if (lstat(state->dir, &st) < 0 || !S_ISDIR(st.st_mode))
+		return 0;
+	if (lstat(am_path(state, "last"), &st) || !S_ISREG(st.st_mode))
+		return 0;
+	if (lstat(am_path(state, "next"), &st) || !S_ISREG(st.st_mode))
+		return 0;
+	return 1;
+}
+
+/**
+ * Reads the contents of `file` in the `state` directory into `sb`. Returns the
+ * number of bytes read on success, -1 if the file does not exist. If `trim` is
+ * set, trailing whitespace will be removed.
+ */
+static int read_state_file(struct strbuf *sb, const struct am_state *state,
+			const char *file, int trim)
+{
+	strbuf_reset(sb);
+
+	if (strbuf_read_file(sb, am_path(state, file), 0) >= 0) {
+		if (trim)
+			strbuf_trim(sb);
+
+		return sb->len;
+	}
+
+	if (errno == ENOENT)
+		return -1;
+
+	die_errno(_("could not read '%s'"), am_path(state, file));
+}
+
+/**
+ * Reads a KEY=VALUE shell variable assignment from `fp`, returning the VALUE
+ * as a newly-allocated string. VALUE must be a quoted string, and the KEY must
+ * match `key`. Returns NULL on failure.
+ *
+ * This is used by read_author_script() to read the GIT_AUTHOR_* variables from
+ * the author-script.
+ */
+static char *read_shell_var(FILE *fp, const char *key)
+{
+	struct strbuf sb = STRBUF_INIT;
+	const char *str;
+
+	if (strbuf_getline(&sb, fp, '\n'))
+		goto fail;
+
+	if (!skip_prefix(sb.buf, key, &str))
+		goto fail;
+
+	if (!skip_prefix(str, "=", &str))
+		goto fail;
+
+	strbuf_remove(&sb, 0, str - sb.buf);
+
+	str = sq_dequote(sb.buf);
+	if (!str)
+		goto fail;
+
+	return strbuf_detach(&sb, NULL);
+
+fail:
+	strbuf_release(&sb);
+	return NULL;
+}
+
+/**
+ * Reads and parses the state directory's "author-script" file, and sets
+ * state->author_name, state->author_email and state->author_date accordingly.
+ * Returns 0 on success, -1 if the file could not be parsed.
+ *
+ * The author script is of the format:
+ *
+ *	GIT_AUTHOR_NAME='$author_name'
+ *	GIT_AUTHOR_EMAIL='$author_email'
+ *	GIT_AUTHOR_DATE='$author_date'
+ *
+ * where $author_name, $author_email and $author_date are quoted. We are strict
+ * with our parsing, as the file was meant to be eval'd in the old git-am.sh
+ * script, and thus if the file differs from what this function expects, it is
+ * better to bail out than to do something that the user does not expect.
+ */
+static int read_author_script(struct am_state *state)
+{
+	const char *filename = am_path(state, "author-script");
+	FILE *fp;
+
+	assert(!state->author_name);
+	assert(!state->author_email);
+	assert(!state->author_date);
+
+	fp = fopen(filename, "r");
+	if (!fp) {
+		if (errno == ENOENT)
+			return 0;
+		die_errno(_("could not open '%s' for reading"), filename);
+	}
+
+	state->author_name = read_shell_var(fp, "GIT_AUTHOR_NAME");
+	if (!state->author_name) {
+		fclose(fp);
+		return -1;
+	}
+
+	state->author_email = read_shell_var(fp, "GIT_AUTHOR_EMAIL");
+	if (!state->author_email) {
+		fclose(fp);
+		return -1;
+	}
+
+	state->author_date = read_shell_var(fp, "GIT_AUTHOR_DATE");
+	if (!state->author_date) {
+		fclose(fp);
+		return -1;
+	}
+
+	if (fgetc(fp) != EOF) {
+		fclose(fp);
+		return -1;
+	}
+
+	fclose(fp);
+	return 0;
+}
+
+/**
+ * Saves state->author_name, state->author_email and state->author_date in the
+ * state directory's "author-script" file.
+ */
+static void write_author_script(const struct am_state *state)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	strbuf_addstr(&sb, "GIT_AUTHOR_NAME=");
+	sq_quote_buf(&sb, state->author_name);
+	strbuf_addch(&sb, '\n');
+
+	strbuf_addstr(&sb, "GIT_AUTHOR_EMAIL=");
+	sq_quote_buf(&sb, state->author_email);
+	strbuf_addch(&sb, '\n');
+
+	strbuf_addstr(&sb, "GIT_AUTHOR_DATE=");
+	sq_quote_buf(&sb, state->author_date);
+	strbuf_addch(&sb, '\n');
+
+	write_file(am_path(state, "author-script"), 1, "%s", sb.buf);
+
+	strbuf_release(&sb);
+}
+
+/**
+ * Reads the commit message from the state directory's "final-commit" file,
+ * setting state->msg to its contents and state->msg_len to the length of its
+ * contents in bytes.
+ *
+ * Returns 0 on success, -1 if the file does not exist.
+ */
+static int read_commit_msg(struct am_state *state)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	assert(!state->msg);
+
+	if (read_state_file(&sb, state, "final-commit", 0) < 0) {
+		strbuf_release(&sb);
+		return -1;
+	}
+
+	state->msg = strbuf_detach(&sb, &state->msg_len);
+	return 0;
+}
+
+/**
+ * Saves state->msg in the state directory's "final-commit" file.
+ */
+static void write_commit_msg(const struct am_state *state)
+{
+	int fd;
+	const char *filename = am_path(state, "final-commit");
+
+	fd = xopen(filename, O_WRONLY | O_CREAT, 0666);
+	if (write_in_full(fd, state->msg, state->msg_len) < 0)
+		die_errno(_("could not write to %s"), filename);
+	close(fd);
+}
+
+/**
+ * Loads state from disk.
+ */
+static void am_load(struct am_state *state)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	if (read_state_file(&sb, state, "next", 1) < 0)
+		die("BUG: state file 'next' does not exist");
+	state->cur = strtol(sb.buf, NULL, 10);
+
+	if (read_state_file(&sb, state, "last", 1) < 0)
+		die("BUG: state file 'last' does not exist");
+	state->last = strtol(sb.buf, NULL, 10);
+
+	if (read_author_script(state) < 0)
+		die(_("could not parse author script"));
+
+	read_commit_msg(state);
+
+	if (read_state_file(&sb, state, "original-commit", 1) < 0)
+		hashclr(state->orig_commit);
+	else if (get_sha1_hex(sb.buf, state->orig_commit) < 0)
+		die(_("could not parse %s"), am_path(state, "original-commit"));
+
+	read_state_file(&sb, state, "threeway", 1);
+	state->threeway = !strcmp(sb.buf, "t");
+
+	read_state_file(&sb, state, "quiet", 1);
+	state->quiet = !strcmp(sb.buf, "t");
+
+	read_state_file(&sb, state, "sign", 1);
+	state->signoff = !strcmp(sb.buf, "t");
+
+	read_state_file(&sb, state, "utf8", 1);
+	state->utf8 = !strcmp(sb.buf, "t");
+
+	read_state_file(&sb, state, "keep", 1);
+	if (!strcmp(sb.buf, "t"))
+		state->keep = KEEP_TRUE;
+	else if (!strcmp(sb.buf, "b"))
+		state->keep = KEEP_NON_PATCH;
+	else
+		state->keep = KEEP_FALSE;
+
+	read_state_file(&sb, state, "messageid", 1);
+	state->message_id = !strcmp(sb.buf, "t");
+
+	read_state_file(&sb, state, "scissors", 1);
+	if (!strcmp(sb.buf, "t"))
+		state->scissors = SCISSORS_TRUE;
+	else if (!strcmp(sb.buf, "f"))
+		state->scissors = SCISSORS_FALSE;
+	else
+		state->scissors = SCISSORS_UNSET;
+
+	read_state_file(&sb, state, "apply-opt", 1);
+	argv_array_clear(&state->git_apply_opts);
+	if (sq_dequote_to_argv_array(sb.buf, &state->git_apply_opts) < 0)
+		die(_("could not parse %s"), am_path(state, "apply-opt"));
+
+	state->rebasing = !!file_exists(am_path(state, "rebasing"));
+
+	strbuf_release(&sb);
+}
+
+/**
+ * Removes the am_state directory, forcefully terminating the current am
+ * session.
+ */
+static void am_destroy(const struct am_state *state)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	strbuf_addstr(&sb, state->dir);
+	remove_dir_recursively(&sb, 0);
+	strbuf_release(&sb);
+}
+
+/**
+ * Runs applypatch-msg hook. Returns its exit code.
+ */
+static int run_applypatch_msg_hook(struct am_state *state)
+{
+	int ret;
+
+	assert(state->msg);
+	ret = run_hook_le(NULL, "applypatch-msg", am_path(state, "final-commit"), NULL);
+
+	if (!ret) {
+		free(state->msg);
+		state->msg = NULL;
+		if (read_commit_msg(state) < 0)
+			die(_("'%s' was deleted by the applypatch-msg hook"),
+				am_path(state, "final-commit"));
+	}
+
+	return ret;
+}
+
+/**
+ * Runs post-rewrite hook. Returns it exit code.
+ */
+static int run_post_rewrite_hook(const struct am_state *state)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	const char *hook = find_hook("post-rewrite");
+	int ret;
+
+	if (!hook)
+		return 0;
+
+	argv_array_push(&cp.args, hook);
+	argv_array_push(&cp.args, "rebase");
+
+	cp.in = xopen(am_path(state, "rewritten"), O_RDONLY);
+	cp.stdout_to_stderr = 1;
+
+	ret = run_command(&cp);
+
+	close(cp.in);
+	return ret;
+}
+
+/**
+ * Reads the state directory's "rewritten" file, and copies notes from the old
+ * commits listed in the file to their rewritten commits.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+static int copy_notes_for_rebase(const struct am_state *state)
+{
+	struct notes_rewrite_cfg *c;
+	struct strbuf sb = STRBUF_INIT;
+	const char *invalid_line = _("Malformed input line: '%s'.");
+	const char *msg = "Notes added by 'git rebase'";
+	FILE *fp;
+	int ret = 0;
+
+	assert(state->rebasing);
+
+	c = init_copy_notes_for_rewrite("rebase");
+	if (!c)
+		return 0;
+
+	fp = xfopen(am_path(state, "rewritten"), "r");
+
+	while (!strbuf_getline(&sb, fp, '\n')) {
+		unsigned char from_obj[GIT_SHA1_RAWSZ], to_obj[GIT_SHA1_RAWSZ];
+
+		if (sb.len != GIT_SHA1_HEXSZ * 2 + 1) {
+			ret = error(invalid_line, sb.buf);
+			goto finish;
+		}
+
+		if (get_sha1_hex(sb.buf, from_obj)) {
+			ret = error(invalid_line, sb.buf);
+			goto finish;
+		}
+
+		if (sb.buf[GIT_SHA1_HEXSZ] != ' ') {
+			ret = error(invalid_line, sb.buf);
+			goto finish;
+		}
+
+		if (get_sha1_hex(sb.buf + GIT_SHA1_HEXSZ + 1, to_obj)) {
+			ret = error(invalid_line, sb.buf);
+			goto finish;
+		}
+
+		if (copy_note_for_rewrite(c, from_obj, to_obj))
+			ret = error(_("Failed to copy notes from '%s' to '%s'"),
+					sha1_to_hex(from_obj), sha1_to_hex(to_obj));
+	}
+
+finish:
+	finish_copy_notes_for_rewrite(c, msg);
+	fclose(fp);
+	strbuf_release(&sb);
+	return ret;
+}
+
+/**
+ * Determines if the file looks like a piece of RFC2822 mail by grabbing all
+ * non-indented lines and checking if they look like they begin with valid
+ * header field names.
+ *
+ * Returns 1 if the file looks like a piece of mail, 0 otherwise.
+ */
+static int is_mail(FILE *fp)
+{
+	const char *header_regex = "^[!-9;-~]+:";
+	struct strbuf sb = STRBUF_INIT;
+	regex_t regex;
+	int ret = 1;
+
+	if (fseek(fp, 0L, SEEK_SET))
+		die_errno(_("fseek failed"));
+
+	if (regcomp(&regex, header_regex, REG_NOSUB | REG_EXTENDED))
+		die("invalid pattern: %s", header_regex);
+
+	while (!strbuf_getline_crlf(&sb, fp)) {
+		if (!sb.len)
+			break; /* End of header */
+
+		/* Ignore indented folded lines */
+		if (*sb.buf == '\t' || *sb.buf == ' ')
+			continue;
+
+		/* It's a header if it matches header_regex */
+		if (regexec(&regex, sb.buf, 0, NULL, 0)) {
+			ret = 0;
+			goto done;
+		}
+	}
+
+done:
+	regfree(&regex);
+	strbuf_release(&sb);
+	return ret;
+}
+
+/**
+ * Attempts to detect the patch_format of the patches contained in `paths`,
+ * returning the PATCH_FORMAT_* enum value. Returns PATCH_FORMAT_UNKNOWN if
+ * detection fails.
+ */
+static int detect_patch_format(const char **paths)
+{
+	enum patch_format ret = PATCH_FORMAT_UNKNOWN;
+	struct strbuf l1 = STRBUF_INIT;
+	struct strbuf l2 = STRBUF_INIT;
+	struct strbuf l3 = STRBUF_INIT;
+	FILE *fp;
+
+	/*
+	 * We default to mbox format if input is from stdin and for directories
+	 */
+	if (!*paths || !strcmp(*paths, "-") || is_directory(*paths))
+		return PATCH_FORMAT_MBOX;
+
+	/*
+	 * Otherwise, check the first few lines of the first patch, starting
+	 * from the first non-blank line, to try to detect its format.
+	 */
+
+	fp = xfopen(*paths, "r");
+
+	while (!strbuf_getline_crlf(&l1, fp)) {
+		if (l1.len)
+			break;
+	}
+
+	if (starts_with(l1.buf, "From ") || starts_with(l1.buf, "From: ")) {
+		ret = PATCH_FORMAT_MBOX;
+		goto done;
+	}
+
+	if (starts_with(l1.buf, "# This series applies on GIT commit")) {
+		ret = PATCH_FORMAT_STGIT_SERIES;
+		goto done;
+	}
+
+	if (!strcmp(l1.buf, "# HG changeset patch")) {
+		ret = PATCH_FORMAT_HG;
+		goto done;
+	}
+
+	strbuf_reset(&l2);
+	strbuf_getline_crlf(&l2, fp);
+	strbuf_reset(&l3);
+	strbuf_getline_crlf(&l3, fp);
+
+	/*
+	 * If the second line is empty and the third is a From, Author or Date
+	 * entry, this is likely an StGit patch.
+	 */
+	if (l1.len && !l2.len &&
+		(starts_with(l3.buf, "From:") ||
+		 starts_with(l3.buf, "Author:") ||
+		 starts_with(l3.buf, "Date:"))) {
+		ret = PATCH_FORMAT_STGIT;
+		goto done;
+	}
+
+	if (l1.len && is_mail(fp)) {
+		ret = PATCH_FORMAT_MBOX;
+		goto done;
+	}
+
+done:
+	fclose(fp);
+	strbuf_release(&l1);
+	return ret;
+}
+
+/**
+ * Splits out individual email patches from `paths`, where each path is either
+ * a mbox file or a Maildir. Returns 0 on success, -1 on failure.
+ */
+static int split_mail_mbox(struct am_state *state, const char **paths, int keep_cr)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	struct strbuf last = STRBUF_INIT;
+
+	cp.git_cmd = 1;
+	argv_array_push(&cp.args, "mailsplit");
+	argv_array_pushf(&cp.args, "-d%d", state->prec);
+	argv_array_pushf(&cp.args, "-o%s", state->dir);
+	argv_array_push(&cp.args, "-b");
+	if (keep_cr)
+		argv_array_push(&cp.args, "--keep-cr");
+	argv_array_push(&cp.args, "--");
+	argv_array_pushv(&cp.args, paths);
+
+	if (capture_command(&cp, &last, 8))
+		return -1;
+
+	state->cur = 1;
+	state->last = strtol(last.buf, NULL, 10);
+
+	return 0;
+}
+
+/**
+ * Callback signature for split_mail_conv(). The foreign patch should be
+ * read from `in`, and the converted patch (in RFC2822 mail format) should be
+ * written to `out`. Return 0 on success, or -1 on failure.
+ */
+typedef int (*mail_conv_fn)(FILE *out, FILE *in, int keep_cr);
+
+/**
+ * Calls `fn` for each file in `paths` to convert the foreign patch to the
+ * RFC2822 mail format suitable for parsing with git-mailinfo.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+static int split_mail_conv(mail_conv_fn fn, struct am_state *state,
+			const char **paths, int keep_cr)
+{
+	static const char *stdin_only[] = {"-", NULL};
+	int i;
+
+	if (!*paths)
+		paths = stdin_only;
+
+	for (i = 0; *paths; paths++, i++) {
+		FILE *in, *out;
+		const char *mail;
+		int ret;
+
+		if (!strcmp(*paths, "-"))
+			in = stdin;
+		else
+			in = fopen(*paths, "r");
+
+		if (!in)
+			return error(_("could not open '%s' for reading: %s"),
+					*paths, strerror(errno));
+
+		mail = mkpath("%s/%0*d", state->dir, state->prec, i + 1);
+
+		out = fopen(mail, "w");
+		if (!out)
+			return error(_("could not open '%s' for writing: %s"),
+					mail, strerror(errno));
+
+		ret = fn(out, in, keep_cr);
+
+		fclose(out);
+		fclose(in);
+
+		if (ret)
+			return error(_("could not parse patch '%s'"), *paths);
+	}
+
+	state->cur = 1;
+	state->last = i;
+	return 0;
+}
+
+/**
+ * A split_mail_conv() callback that converts an StGit patch to an RFC2822
+ * message suitable for parsing with git-mailinfo.
+ */
+static int stgit_patch_to_mail(FILE *out, FILE *in, int keep_cr)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int subject_printed = 0;
+
+	while (!strbuf_getline(&sb, in, '\n')) {
+		const char *str;
+
+		if (str_isspace(sb.buf))
+			continue;
+		else if (skip_prefix(sb.buf, "Author:", &str))
+			fprintf(out, "From:%s\n", str);
+		else if (starts_with(sb.buf, "From") || starts_with(sb.buf, "Date"))
+			fprintf(out, "%s\n", sb.buf);
+		else if (!subject_printed) {
+			fprintf(out, "Subject: %s\n", sb.buf);
+			subject_printed = 1;
+		} else {
+			fprintf(out, "\n%s\n", sb.buf);
+			break;
+		}
+	}
+
+	strbuf_reset(&sb);
+	while (strbuf_fread(&sb, 8192, in) > 0) {
+		fwrite(sb.buf, 1, sb.len, out);
+		strbuf_reset(&sb);
+	}
+
+	strbuf_release(&sb);
+	return 0;
+}
+
+/**
+ * This function only supports a single StGit series file in `paths`.
+ *
+ * Given an StGit series file, converts the StGit patches in the series into
+ * RFC2822 messages suitable for parsing with git-mailinfo, and queues them in
+ * the state directory.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+static int split_mail_stgit_series(struct am_state *state, const char **paths,
+					int keep_cr)
+{
+	const char *series_dir;
+	char *series_dir_buf;
+	FILE *fp;
+	struct argv_array patches = ARGV_ARRAY_INIT;
+	struct strbuf sb = STRBUF_INIT;
+	int ret;
+
+	if (!paths[0] || paths[1])
+		return error(_("Only one StGIT patch series can be applied at once"));
+
+	series_dir_buf = xstrdup(*paths);
+	series_dir = dirname(series_dir_buf);
+
+	fp = fopen(*paths, "r");
+	if (!fp)
+		return error(_("could not open '%s' for reading: %s"), *paths,
+				strerror(errno));
+
+	while (!strbuf_getline(&sb, fp, '\n')) {
+		if (*sb.buf == '#')
+			continue; /* skip comment lines */
+
+		argv_array_push(&patches, mkpath("%s/%s", series_dir, sb.buf));
+	}
+
+	fclose(fp);
+	strbuf_release(&sb);
+	free(series_dir_buf);
+
+	ret = split_mail_conv(stgit_patch_to_mail, state, patches.argv, keep_cr);
+
+	argv_array_clear(&patches);
+	return ret;
+}
+
+/**
+ * A split_patches_conv() callback that converts a mercurial patch to a RFC2822
+ * message suitable for parsing with git-mailinfo.
+ */
+static int hg_patch_to_mail(FILE *out, FILE *in, int keep_cr)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	while (!strbuf_getline(&sb, in, '\n')) {
+		const char *str;
+
+		if (skip_prefix(sb.buf, "# User ", &str))
+			fprintf(out, "From: %s\n", str);
+		else if (skip_prefix(sb.buf, "# Date ", &str)) {
+			unsigned long timestamp;
+			long tz, tz2;
+			char *end;
+
+			errno = 0;
+			timestamp = strtoul(str, &end, 10);
+			if (errno)
+				return error(_("invalid timestamp"));
+
+			if (!skip_prefix(end, " ", &str))
+				return error(_("invalid Date line"));
+
+			errno = 0;
+			tz = strtol(str, &end, 10);
+			if (errno)
+				return error(_("invalid timezone offset"));
+
+			if (*end)
+				return error(_("invalid Date line"));
+
+			/*
+			 * mercurial's timezone is in seconds west of UTC,
+			 * however git's timezone is in hours + minutes east of
+			 * UTC. Convert it.
+			 */
+			tz2 = labs(tz) / 3600 * 100 + labs(tz) % 3600 / 60;
+			if (tz > 0)
+				tz2 = -tz2;
+
+			fprintf(out, "Date: %s\n", show_date(timestamp, tz2, DATE_MODE(RFC2822)));
+		} else if (starts_with(sb.buf, "# ")) {
+			continue;
+		} else {
+			fprintf(out, "\n%s\n", sb.buf);
+			break;
+		}
+	}
+
+	strbuf_reset(&sb);
+	while (strbuf_fread(&sb, 8192, in) > 0) {
+		fwrite(sb.buf, 1, sb.len, out);
+		strbuf_reset(&sb);
+	}
+
+	strbuf_release(&sb);
+	return 0;
+}
+
+/**
+ * Splits a list of files/directories into individual email patches. Each path
+ * in `paths` must be a file/directory that is formatted according to
+ * `patch_format`.
+ *
+ * Once split out, the individual email patches will be stored in the state
+ * directory, with each patch's filename being its index, padded to state->prec
+ * digits.
+ *
+ * state->cur will be set to the index of the first mail, and state->last will
+ * be set to the index of the last mail.
+ *
+ * Set keep_cr to 0 to convert all lines ending with \r\n to end with \n, 1
+ * to disable this behavior, -1 to use the default configured setting.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+static int split_mail(struct am_state *state, enum patch_format patch_format,
+			const char **paths, int keep_cr)
+{
+	if (keep_cr < 0) {
+		keep_cr = 0;
+		git_config_get_bool("am.keepcr", &keep_cr);
+	}
+
+	switch (patch_format) {
+	case PATCH_FORMAT_MBOX:
+		return split_mail_mbox(state, paths, keep_cr);
+	case PATCH_FORMAT_STGIT:
+		return split_mail_conv(stgit_patch_to_mail, state, paths, keep_cr);
+	case PATCH_FORMAT_STGIT_SERIES:
+		return split_mail_stgit_series(state, paths, keep_cr);
+	case PATCH_FORMAT_HG:
+		return split_mail_conv(hg_patch_to_mail, state, paths, keep_cr);
+	default:
+		die("BUG: invalid patch_format");
+	}
+	return -1;
+}
+
+/**
+ * Setup a new am session for applying patches
+ */
+static void am_setup(struct am_state *state, enum patch_format patch_format,
+			const char **paths, int keep_cr)
+{
+	unsigned char curr_head[GIT_SHA1_RAWSZ];
+	const char *str;
+	struct strbuf sb = STRBUF_INIT;
+
+	if (!patch_format)
+		patch_format = detect_patch_format(paths);
+
+	if (!patch_format) {
+		fprintf_ln(stderr, _("Patch format detection failed."));
+		exit(128);
+	}
+
+	if (mkdir(state->dir, 0777) < 0 && errno != EEXIST)
+		die_errno(_("failed to create directory '%s'"), state->dir);
+
+	if (split_mail(state, patch_format, paths, keep_cr) < 0) {
+		am_destroy(state);
+		die(_("Failed to split patches."));
+	}
+
+	if (state->rebasing)
+		state->threeway = 1;
+
+	write_file(am_path(state, "threeway"), 1, state->threeway ? "t" : "f");
+
+	write_file(am_path(state, "quiet"), 1, state->quiet ? "t" : "f");
+
+	write_file(am_path(state, "sign"), 1, state->signoff ? "t" : "f");
+
+	write_file(am_path(state, "utf8"), 1, state->utf8 ? "t" : "f");
+
+	switch (state->keep) {
+	case KEEP_FALSE:
+		str = "f";
+		break;
+	case KEEP_TRUE:
+		str = "t";
+		break;
+	case KEEP_NON_PATCH:
+		str = "b";
+		break;
+	default:
+		die("BUG: invalid value for state->keep");
+	}
+
+	write_file(am_path(state, "keep"), 1, "%s", str);
+
+	write_file(am_path(state, "messageid"), 1, state->message_id ? "t" : "f");
+
+	switch (state->scissors) {
+	case SCISSORS_UNSET:
+		str = "";
+		break;
+	case SCISSORS_FALSE:
+		str = "f";
+		break;
+	case SCISSORS_TRUE:
+		str = "t";
+		break;
+	default:
+		die("BUG: invalid value for state->scissors");
+	}
+
+	write_file(am_path(state, "scissors"), 1, "%s", str);
+
+	sq_quote_argv(&sb, state->git_apply_opts.argv, 0);
+	write_file(am_path(state, "apply-opt"), 1, "%s", sb.buf);
+
+	if (state->rebasing)
+		write_file(am_path(state, "rebasing"), 1, "%s", "");
+	else
+		write_file(am_path(state, "applying"), 1, "%s", "");
+
+	if (!get_sha1("HEAD", curr_head)) {
+		write_file(am_path(state, "abort-safety"), 1, "%s", sha1_to_hex(curr_head));
+		if (!state->rebasing)
+			update_ref("am", "ORIG_HEAD", curr_head, NULL, 0,
+					UPDATE_REFS_DIE_ON_ERR);
+	} else {
+		write_file(am_path(state, "abort-safety"), 1, "%s", "");
+		if (!state->rebasing)
+			delete_ref("ORIG_HEAD", NULL, 0);
+	}
+
+	/*
+	 * NOTE: Since the "next" and "last" files determine if an am_state
+	 * session is in progress, they should be written last.
+	 */
+
+	write_file(am_path(state, "next"), 1, "%d", state->cur);
+
+	write_file(am_path(state, "last"), 1, "%d", state->last);
+
+	strbuf_release(&sb);
+}
+
+/**
+ * Increments the patch pointer, and cleans am_state for the application of the
+ * next patch.
+ */
+static void am_next(struct am_state *state)
+{
+	unsigned char head[GIT_SHA1_RAWSZ];
+
+	free(state->author_name);
+	state->author_name = NULL;
+
+	free(state->author_email);
+	state->author_email = NULL;
+
+	free(state->author_date);
+	state->author_date = NULL;
+
+	free(state->msg);
+	state->msg = NULL;
+	state->msg_len = 0;
+
+	unlink(am_path(state, "author-script"));
+	unlink(am_path(state, "final-commit"));
+
+	hashclr(state->orig_commit);
+	unlink(am_path(state, "original-commit"));
+
+	if (!get_sha1("HEAD", head))
+		write_file(am_path(state, "abort-safety"), 1, "%s", sha1_to_hex(head));
+	else
+		write_file(am_path(state, "abort-safety"), 1, "%s", "");
+
+	state->cur++;
+	write_file(am_path(state, "next"), 1, "%d", state->cur);
+}
+
+/**
+ * Returns the filename of the current patch email.
+ */
+static const char *msgnum(const struct am_state *state)
+{
+	static struct strbuf sb = STRBUF_INIT;
+
+	strbuf_reset(&sb);
+	strbuf_addf(&sb, "%0*d", state->prec, state->cur);
+
+	return sb.buf;
+}
+
+/**
+ * Refresh and write index.
+ */
+static void refresh_and_write_cache(void)
+{
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+
+	hold_locked_index(lock_file, 1);
+	refresh_cache(REFRESH_QUIET);
+	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+		die(_("unable to write index file"));
+}
+
+/**
+ * Returns 1 if the index differs from HEAD, 0 otherwise. When on an unborn
+ * branch, returns 1 if there are entries in the index, 0 otherwise. If an
+ * strbuf is provided, the space-separated list of files that differ will be
+ * appended to it.
+ */
+static int index_has_changes(struct strbuf *sb)
+{
+	unsigned char head[GIT_SHA1_RAWSZ];
+	int i;
+
+	if (!get_sha1_tree("HEAD", head)) {
+		struct diff_options opt;
+
+		diff_setup(&opt);
+		DIFF_OPT_SET(&opt, EXIT_WITH_STATUS);
+		if (!sb)
+			DIFF_OPT_SET(&opt, QUICK);
+		do_diff_cache(head, &opt);
+		diffcore_std(&opt);
+		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
+			if (i)
+				strbuf_addch(sb, ' ');
+			strbuf_addstr(sb, diff_queued_diff.queue[i]->two->path);
+		}
+		diff_flush(&opt);
+		return DIFF_OPT_TST(&opt, HAS_CHANGES) != 0;
+	} else {
+		for (i = 0; sb && i < active_nr; i++) {
+			if (i)
+				strbuf_addch(sb, ' ');
+			strbuf_addstr(sb, active_cache[i]->name);
+		}
+		return !!active_nr;
+	}
+}
+
+/**
+ * Dies with a user-friendly message on how to proceed after resolving the
+ * problem. This message can be overridden with state->resolvemsg.
+ */
+static void NORETURN die_user_resolve(const struct am_state *state)
+{
+	if (state->resolvemsg) {
+		printf_ln("%s", state->resolvemsg);
+	} else {
+		const char *cmdline = state->interactive ? "git am -i" : "git am";
+
+		printf_ln(_("When you have resolved this problem, run \"%s --continue\"."), cmdline);
+		printf_ln(_("If you prefer to skip this patch, run \"%s --skip\" instead."), cmdline);
+		printf_ln(_("To restore the original branch and stop patching, run \"%s --abort\"."), cmdline);
+	}
+
+	exit(128);
+}
+
+/**
+ * Parses `mail` using git-mailinfo, extracting its patch and authorship info.
+ * state->msg will be set to the patch message. state->author_name,
+ * state->author_email and state->author_date will be set to the patch author's
+ * name, email and date respectively. The patch body will be written to the
+ * state directory's "patch" file.
+ *
+ * Returns 1 if the patch should be skipped, 0 otherwise.
+ */
+static int parse_mail(struct am_state *state, const char *mail)
+{
+	FILE *fp;
+	struct child_process cp = CHILD_PROCESS_INIT;
+	struct strbuf sb = STRBUF_INIT;
+	struct strbuf msg = STRBUF_INIT;
+	struct strbuf author_name = STRBUF_INIT;
+	struct strbuf author_date = STRBUF_INIT;
+	struct strbuf author_email = STRBUF_INIT;
+	int ret = 0;
+
+	cp.git_cmd = 1;
+	cp.in = xopen(mail, O_RDONLY, 0);
+	cp.out = xopen(am_path(state, "info"), O_WRONLY | O_CREAT, 0777);
+
+	argv_array_push(&cp.args, "mailinfo");
+	argv_array_push(&cp.args, state->utf8 ? "-u" : "-n");
+
+	switch (state->keep) {
+	case KEEP_FALSE:
+		break;
+	case KEEP_TRUE:
+		argv_array_push(&cp.args, "-k");
+		break;
+	case KEEP_NON_PATCH:
+		argv_array_push(&cp.args, "-b");
+		break;
+	default:
+		die("BUG: invalid value for state->keep");
+	}
+
+	if (state->message_id)
+		argv_array_push(&cp.args, "-m");
+
+	switch (state->scissors) {
+	case SCISSORS_UNSET:
+		break;
+	case SCISSORS_FALSE:
+		argv_array_push(&cp.args, "--no-scissors");
+		break;
+	case SCISSORS_TRUE:
+		argv_array_push(&cp.args, "--scissors");
+		break;
+	default:
+		die("BUG: invalid value for state->scissors");
+	}
+
+	argv_array_push(&cp.args, am_path(state, "msg"));
+	argv_array_push(&cp.args, am_path(state, "patch"));
+
+	if (run_command(&cp) < 0)
+		die("could not parse patch");
+
+	close(cp.in);
+	close(cp.out);
+
+	/* Extract message and author information */
+	fp = xfopen(am_path(state, "info"), "r");
+	while (!strbuf_getline(&sb, fp, '\n')) {
+		const char *x;
+
+		if (skip_prefix(sb.buf, "Subject: ", &x)) {
+			if (msg.len)
+				strbuf_addch(&msg, '\n');
+			strbuf_addstr(&msg, x);
+		} else if (skip_prefix(sb.buf, "Author: ", &x))
+			strbuf_addstr(&author_name, x);
+		else if (skip_prefix(sb.buf, "Email: ", &x))
+			strbuf_addstr(&author_email, x);
+		else if (skip_prefix(sb.buf, "Date: ", &x))
+			strbuf_addstr(&author_date, x);
+	}
+	fclose(fp);
+
+	/* Skip pine's internal folder data */
+	if (!strcmp(author_name.buf, "Mail System Internal Data")) {
+		ret = 1;
+		goto finish;
+	}
+
+	if (is_empty_file(am_path(state, "patch"))) {
+		printf_ln(_("Patch is empty. Was it split wrong?"));
+		die_user_resolve(state);
+	}
+
+	strbuf_addstr(&msg, "\n\n");
+	if (strbuf_read_file(&msg, am_path(state, "msg"), 0) < 0)
+		die_errno(_("could not read '%s'"), am_path(state, "msg"));
+	stripspace(&msg, 0);
+
+	if (state->signoff)
+		append_signoff(&msg, 0, 0);
+
+	assert(!state->author_name);
+	state->author_name = strbuf_detach(&author_name, NULL);
+
+	assert(!state->author_email);
+	state->author_email = strbuf_detach(&author_email, NULL);
+
+	assert(!state->author_date);
+	state->author_date = strbuf_detach(&author_date, NULL);
+
+	assert(!state->msg);
+	state->msg = strbuf_detach(&msg, &state->msg_len);
+
+finish:
+	strbuf_release(&msg);
+	strbuf_release(&author_date);
+	strbuf_release(&author_email);
+	strbuf_release(&author_name);
+	strbuf_release(&sb);
+	return ret;
+}
+
+/**
+ * Sets commit_id to the commit hash where the mail was generated from.
+ * Returns 0 on success, -1 on failure.
+ */
+static int get_mail_commit_sha1(unsigned char *commit_id, const char *mail)
+{
+	struct strbuf sb = STRBUF_INIT;
+	FILE *fp = xfopen(mail, "r");
+	const char *x;
+
+	if (strbuf_getline(&sb, fp, '\n'))
+		return -1;
+
+	if (!skip_prefix(sb.buf, "From ", &x))
+		return -1;
+
+	if (get_sha1_hex(x, commit_id) < 0)
+		return -1;
+
+	strbuf_release(&sb);
+	fclose(fp);
+	return 0;
+}
+
+/**
+ * Sets state->msg, state->author_name, state->author_email, state->author_date
+ * to the commit's respective info.
+ */
+static void get_commit_info(struct am_state *state, struct commit *commit)
+{
+	const char *buffer, *ident_line, *author_date, *msg;
+	size_t ident_len;
+	struct ident_split ident_split;
+	struct strbuf sb = STRBUF_INIT;
+
+	buffer = logmsg_reencode(commit, NULL, get_commit_output_encoding());
+
+	ident_line = find_commit_header(buffer, "author", &ident_len);
+
+	if (split_ident_line(&ident_split, ident_line, ident_len) < 0) {
+		strbuf_add(&sb, ident_line, ident_len);
+		die(_("invalid ident line: %s"), sb.buf);
+	}
+
+	assert(!state->author_name);
+	if (ident_split.name_begin) {
+		strbuf_add(&sb, ident_split.name_begin,
+			ident_split.name_end - ident_split.name_begin);
+		state->author_name = strbuf_detach(&sb, NULL);
+	} else
+		state->author_name = xstrdup("");
+
+	assert(!state->author_email);
+	if (ident_split.mail_begin) {
+		strbuf_add(&sb, ident_split.mail_begin,
+			ident_split.mail_end - ident_split.mail_begin);
+		state->author_email = strbuf_detach(&sb, NULL);
+	} else
+		state->author_email = xstrdup("");
+
+	author_date = show_ident_date(&ident_split, DATE_MODE(NORMAL));
+	strbuf_addstr(&sb, author_date);
+	assert(!state->author_date);
+	state->author_date = strbuf_detach(&sb, NULL);
+
+	assert(!state->msg);
+	msg = strstr(buffer, "\n\n");
+	if (!msg)
+		die(_("unable to parse commit %s"), sha1_to_hex(commit->object.sha1));
+	state->msg = xstrdup(msg + 2);
+	state->msg_len = strlen(state->msg);
+}
+
+/**
+ * Writes `commit` as a patch to the state directory's "patch" file.
+ */
+static void write_commit_patch(const struct am_state *state, struct commit *commit)
+{
+	struct rev_info rev_info;
+	FILE *fp;
+
+	fp = xfopen(am_path(state, "patch"), "w");
+	init_revisions(&rev_info, NULL);
+	rev_info.diff = 1;
+	rev_info.abbrev = 0;
+	rev_info.disable_stdin = 1;
+	rev_info.show_root_diff = 1;
+	rev_info.diffopt.output_format = DIFF_FORMAT_PATCH;
+	rev_info.no_commit_id = 1;
+	DIFF_OPT_SET(&rev_info.diffopt, BINARY);
+	DIFF_OPT_SET(&rev_info.diffopt, FULL_INDEX);
+	rev_info.diffopt.use_color = 0;
+	rev_info.diffopt.file = fp;
+	rev_info.diffopt.close_file = 1;
+	add_pending_object(&rev_info, &commit->object, "");
+	diff_setup_done(&rev_info.diffopt);
+	log_tree_commit(&rev_info, commit);
+}
+
+/**
+ * Writes the diff of the index against HEAD as a patch to the state
+ * directory's "patch" file.
+ */
+static void write_index_patch(const struct am_state *state)
+{
+	struct tree *tree;
+	unsigned char head[GIT_SHA1_RAWSZ];
+	struct rev_info rev_info;
+	FILE *fp;
+
+	if (!get_sha1_tree("HEAD", head))
+		tree = lookup_tree(head);
+	else
+		tree = lookup_tree(EMPTY_TREE_SHA1_BIN);
+
+	fp = xfopen(am_path(state, "patch"), "w");
+	init_revisions(&rev_info, NULL);
+	rev_info.diff = 1;
+	rev_info.disable_stdin = 1;
+	rev_info.no_commit_id = 1;
+	rev_info.diffopt.output_format = DIFF_FORMAT_PATCH;
+	rev_info.diffopt.use_color = 0;
+	rev_info.diffopt.file = fp;
+	rev_info.diffopt.close_file = 1;
+	add_pending_object(&rev_info, &tree->object, "");
+	diff_setup_done(&rev_info.diffopt);
+	run_diff_index(&rev_info, 1);
+}
+
+/**
+ * Like parse_mail(), but parses the mail by looking up its commit ID
+ * directly. This is used in --rebasing mode to bypass git-mailinfo's munging
+ * of patches.
+ *
+ * state->orig_commit will be set to the original commit ID.
+ *
+ * Will always return 0 as the patch should never be skipped.
+ */
+static int parse_mail_rebase(struct am_state *state, const char *mail)
+{
+	struct commit *commit;
+	unsigned char commit_sha1[GIT_SHA1_RAWSZ];
+
+	if (get_mail_commit_sha1(commit_sha1, mail) < 0)
+		die(_("could not parse %s"), mail);
+
+	commit = lookup_commit_or_die(commit_sha1, mail);
+
+	get_commit_info(state, commit);
+
+	write_commit_patch(state, commit);
+
+	hashcpy(state->orig_commit, commit_sha1);
+	write_file(am_path(state, "original-commit"), 1, "%s",
+			sha1_to_hex(commit_sha1));
+
+	return 0;
+}
+
+/**
+ * Applies current patch with git-apply. Returns 0 on success, -1 otherwise. If
+ * `index_file` is not NULL, the patch will be applied to that index.
+ */
+static int run_apply(const struct am_state *state, const char *index_file)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+
+	cp.git_cmd = 1;
+
+	if (index_file)
+		argv_array_pushf(&cp.env_array, "GIT_INDEX_FILE=%s", index_file);
+
+	/*
+	 * If we are allowed to fall back on 3-way merge, don't give false
+	 * errors during the initial attempt.
+	 */
+	if (state->threeway && !index_file) {
+		cp.no_stdout = 1;
+		cp.no_stderr = 1;
+	}
+
+	argv_array_push(&cp.args, "apply");
+
+	argv_array_pushv(&cp.args, state->git_apply_opts.argv);
+
+	if (index_file)
+		argv_array_push(&cp.args, "--cached");
+	else
+		argv_array_push(&cp.args, "--index");
+
+	argv_array_push(&cp.args, am_path(state, "patch"));
+
+	if (run_command(&cp))
+		return -1;
+
+	/* Reload index as git-apply will have modified it. */
+	discard_cache();
+	read_cache_from(index_file ? index_file : get_index_file());
+
+	return 0;
+}
+
+/**
+ * Builds an index that contains just the blobs needed for a 3way merge.
+ */
+static int build_fake_ancestor(const struct am_state *state, const char *index_file)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+
+	cp.git_cmd = 1;
+	argv_array_push(&cp.args, "apply");
+	argv_array_pushv(&cp.args, state->git_apply_opts.argv);
+	argv_array_pushf(&cp.args, "--build-fake-ancestor=%s", index_file);
+	argv_array_push(&cp.args, am_path(state, "patch"));
+
+	if (run_command(&cp))
+		return -1;
+
+	return 0;
+}
+
+/**
+ * Attempt a threeway merge, using index_path as the temporary index.
+ */
+static int fall_back_threeway(const struct am_state *state, const char *index_path)
+{
+	unsigned char orig_tree[GIT_SHA1_RAWSZ], his_tree[GIT_SHA1_RAWSZ],
+		      our_tree[GIT_SHA1_RAWSZ];
+	const unsigned char *bases[1] = {orig_tree};
+	struct merge_options o;
+	struct commit *result;
+	char *his_tree_name;
+
+	if (get_sha1("HEAD", our_tree) < 0)
+		hashcpy(our_tree, EMPTY_TREE_SHA1_BIN);
+
+	if (build_fake_ancestor(state, index_path))
+		return error("could not build fake ancestor");
+
+	discard_cache();
+	read_cache_from(index_path);
+
+	if (write_index_as_tree(orig_tree, &the_index, index_path, 0, NULL))
+		return error(_("Repository lacks necessary blobs to fall back on 3-way merge."));
+
+	say(state, stdout, _("Using index info to reconstruct a base tree..."));
+
+	if (!state->quiet) {
+		/*
+		 * List paths that needed 3-way fallback, so that the user can
+		 * review them with extra care to spot mismerges.
+		 */
+		struct rev_info rev_info;
+		const char *diff_filter_str = "--diff-filter=AM";
+
+		init_revisions(&rev_info, NULL);
+		rev_info.diffopt.output_format = DIFF_FORMAT_NAME_STATUS;
+		diff_opt_parse(&rev_info.diffopt, &diff_filter_str, 1);
+		add_pending_sha1(&rev_info, "HEAD", our_tree, 0);
+		diff_setup_done(&rev_info.diffopt);
+		run_diff_index(&rev_info, 1);
+	}
+
+	if (run_apply(state, index_path))
+		return error(_("Did you hand edit your patch?\n"
+				"It does not apply to blobs recorded in its index."));
+
+	if (write_index_as_tree(his_tree, &the_index, index_path, 0, NULL))
+		return error("could not write tree");
+
+	say(state, stdout, _("Falling back to patching base and 3-way merge..."));
+
+	discard_cache();
+	read_cache();
+
+	/*
+	 * This is not so wrong. Depending on which base we picked, orig_tree
+	 * may be wildly different from ours, but his_tree has the same set of
+	 * wildly different changes in parts the patch did not touch, so
+	 * recursive ends up canceling them, saying that we reverted all those
+	 * changes.
+	 */
+
+	init_merge_options(&o);
+
+	o.branch1 = "HEAD";
+	his_tree_name = xstrfmt("%.*s", linelen(state->msg), state->msg);
+	o.branch2 = his_tree_name;
+
+	if (state->quiet)
+		o.verbosity = 0;
+
+	if (merge_recursive_generic(&o, our_tree, his_tree, 1, bases, &result)) {
+		rerere(state->allow_rerere_autoupdate);
+		free(his_tree_name);
+		return error(_("Failed to merge in the changes."));
+	}
+
+	free(his_tree_name);
+	return 0;
+}
+
+/**
+ * Commits the current index with state->msg as the commit message and
+ * state->author_name, state->author_email and state->author_date as the author
+ * information.
+ */
+static void do_commit(const struct am_state *state)
+{
+	unsigned char tree[GIT_SHA1_RAWSZ], parent[GIT_SHA1_RAWSZ],
+		      commit[GIT_SHA1_RAWSZ];
+	unsigned char *ptr;
+	struct commit_list *parents = NULL;
+	const char *reflog_msg, *author;
+	struct strbuf sb = STRBUF_INIT;
+
+	if (run_hook_le(NULL, "pre-applypatch", NULL))
+		exit(1);
+
+	if (write_cache_as_tree(tree, 0, NULL))
+		die(_("git write-tree failed to write a tree"));
+
+	if (!get_sha1_commit("HEAD", parent)) {
+		ptr = parent;
+		commit_list_insert(lookup_commit(parent), &parents);
+	} else {
+		ptr = NULL;
+		say(state, stderr, _("applying to an empty history"));
+	}
+
+	author = fmt_ident(state->author_name, state->author_email,
+			state->ignore_date ? NULL : state->author_date,
+			IDENT_STRICT);
+
+	if (state->committer_date_is_author_date)
+		setenv("GIT_COMMITTER_DATE",
+			state->ignore_date ? "" : state->author_date, 1);
+
+	if (commit_tree(state->msg, state->msg_len, tree, parents, commit,
+				author, state->sign_commit))
+		die(_("failed to write commit object"));
+
+	reflog_msg = getenv("GIT_REFLOG_ACTION");
+	if (!reflog_msg)
+		reflog_msg = "am";
+
+	strbuf_addf(&sb, "%s: %.*s", reflog_msg, linelen(state->msg),
+			state->msg);
+
+	update_ref(sb.buf, "HEAD", commit, ptr, 0, UPDATE_REFS_DIE_ON_ERR);
+
+	if (state->rebasing) {
+		FILE *fp = xfopen(am_path(state, "rewritten"), "a");
+
+		assert(!is_null_sha1(state->orig_commit));
+		fprintf(fp, "%s ", sha1_to_hex(state->orig_commit));
+		fprintf(fp, "%s\n", sha1_to_hex(commit));
+		fclose(fp);
+	}
+
+	run_hook_le(NULL, "post-applypatch", NULL);
+
+	strbuf_release(&sb);
+}
+
+/**
+ * Validates the am_state for resuming -- the "msg" and authorship fields must
+ * be filled up.
+ */
+static void validate_resume_state(const struct am_state *state)
+{
+	if (!state->msg)
+		die(_("cannot resume: %s does not exist."),
+			am_path(state, "final-commit"));
+
+	if (!state->author_name || !state->author_email || !state->author_date)
+		die(_("cannot resume: %s does not exist."),
+			am_path(state, "author-script"));
+}
+
+/**
+ * Interactively prompt the user on whether the current patch should be
+ * applied.
+ *
+ * Returns 0 if the user chooses to apply the patch, 1 if the user chooses to
+ * skip it.
+ */
+static int do_interactive(struct am_state *state)
+{
+	assert(state->msg);
+
+	if (!isatty(0))
+		die(_("cannot be interactive without stdin connected to a terminal."));
+
+	for (;;) {
+		const char *reply;
+
+		puts(_("Commit Body is:"));
+		puts("--------------------------");
+		printf("%s", state->msg);
+		puts("--------------------------");
+
+		/*
+		 * TRANSLATORS: Make sure to include [y], [n], [e], [v] and [a]
+		 * in your translation. The program will only accept English
+		 * input at this point.
+		 */
+		reply = git_prompt(_("Apply? [y]es/[n]o/[e]dit/[v]iew patch/[a]ccept all: "), PROMPT_ECHO);
+
+		if (!reply) {
+			continue;
+		} else if (*reply == 'y' || *reply == 'Y') {
+			return 0;
+		} else if (*reply == 'a' || *reply == 'A') {
+			state->interactive = 0;
+			return 0;
+		} else if (*reply == 'n' || *reply == 'N') {
+			return 1;
+		} else if (*reply == 'e' || *reply == 'E') {
+			struct strbuf msg = STRBUF_INIT;
+
+			if (!launch_editor(am_path(state, "final-commit"), &msg, NULL)) {
+				free(state->msg);
+				state->msg = strbuf_detach(&msg, &state->msg_len);
+			}
+			strbuf_release(&msg);
+		} else if (*reply == 'v' || *reply == 'V') {
+			const char *pager = git_pager(1);
+			struct child_process cp = CHILD_PROCESS_INIT;
+
+			if (!pager)
+				pager = "cat";
+			argv_array_push(&cp.args, pager);
+			argv_array_push(&cp.args, am_path(state, "patch"));
+			run_command(&cp);
+		}
+	}
+}
+
+/**
+ * Applies all queued mail.
+ *
+ * If `resume` is true, we are "resuming". The "msg" and authorship fields, as
+ * well as the state directory's "patch" file is used as-is for applying the
+ * patch and committing it.
+ */
+static void am_run(struct am_state *state, int resume)
+{
+	const char *argv_gc_auto[] = {"gc", "--auto", NULL};
+	struct strbuf sb = STRBUF_INIT;
+
+	unlink(am_path(state, "dirtyindex"));
+
+	refresh_and_write_cache();
+
+	if (index_has_changes(&sb)) {
+		write_file(am_path(state, "dirtyindex"), 1, "t");
+		die(_("Dirty index: cannot apply patches (dirty: %s)"), sb.buf);
+	}
+
+	strbuf_release(&sb);
+
+	while (state->cur <= state->last) {
+		const char *mail = am_path(state, msgnum(state));
+		int apply_status;
+
+		if (!file_exists(mail))
+			goto next;
+
+		if (resume) {
+			validate_resume_state(state);
+			resume = 0;
+		} else {
+			int skip;
+
+			if (state->rebasing)
+				skip = parse_mail_rebase(state, mail);
+			else
+				skip = parse_mail(state, mail);
+
+			if (skip)
+				goto next; /* mail should be skipped */
+
+			write_author_script(state);
+			write_commit_msg(state);
+		}
+
+		if (state->interactive && do_interactive(state))
+			goto next;
+
+		if (run_applypatch_msg_hook(state))
+			exit(1);
+
+		say(state, stdout, _("Applying: %.*s"), linelen(state->msg), state->msg);
+
+		apply_status = run_apply(state, NULL);
+
+		if (apply_status && state->threeway) {
+			struct strbuf sb = STRBUF_INIT;
+
+			strbuf_addstr(&sb, am_path(state, "patch-merge-index"));
+			apply_status = fall_back_threeway(state, sb.buf);
+			strbuf_release(&sb);
+
+			/*
+			 * Applying the patch to an earlier tree and merging
+			 * the result may have produced the same tree as ours.
+			 */
+			if (!apply_status && !index_has_changes(NULL)) {
+				say(state, stdout, _("No changes -- Patch already applied."));
+				goto next;
+			}
+		}
+
+		if (apply_status) {
+			int advice_amworkdir = 1;
+
+			printf_ln(_("Patch failed at %s %.*s"), msgnum(state),
+				linelen(state->msg), state->msg);
+
+			git_config_get_bool("advice.amworkdir", &advice_amworkdir);
+
+			if (advice_amworkdir)
+				printf_ln(_("The copy of the patch that failed is found in: %s"),
+						am_path(state, "patch"));
+
+			die_user_resolve(state);
+		}
+
+		do_commit(state);
+
+next:
+		am_next(state);
+	}
+
+	if (!is_empty_file(am_path(state, "rewritten"))) {
+		assert(state->rebasing);
+		copy_notes_for_rebase(state);
+		run_post_rewrite_hook(state);
+	}
+
+	/*
+	 * In rebasing mode, it's up to the caller to take care of
+	 * housekeeping.
+	 */
+	if (!state->rebasing) {
+		am_destroy(state);
+		run_command_v_opt(argv_gc_auto, RUN_GIT_CMD);
+	}
+}
+
+/**
+ * Resume the current am session after patch application failure. The user did
+ * all the hard work, and we do not have to do any patch application. Just
+ * trust and commit what the user has in the index and working tree.
+ */
+static void am_resolve(struct am_state *state)
+{
+	validate_resume_state(state);
+
+	say(state, stdout, _("Applying: %.*s"), linelen(state->msg), state->msg);
+
+	if (!index_has_changes(NULL)) {
+		printf_ln(_("No changes - did you forget to use 'git add'?\n"
+			"If there is nothing left to stage, chances are that something else\n"
+			"already introduced the same changes; you might want to skip this patch."));
+		die_user_resolve(state);
+	}
+
+	if (unmerged_cache()) {
+		printf_ln(_("You still have unmerged paths in your index.\n"
+			"Did you forget to use 'git add'?"));
+		die_user_resolve(state);
+	}
+
+	if (state->interactive) {
+		write_index_patch(state);
+		if (do_interactive(state))
+			goto next;
+	}
+
+	rerere(0);
+
+	do_commit(state);
+
+next:
+	am_next(state);
+	am_run(state, 0);
+}
+
+/**
+ * Performs a checkout fast-forward from `head` to `remote`. If `reset` is
+ * true, any unmerged entries will be discarded. Returns 0 on success, -1 on
+ * failure.
+ */
+static int fast_forward_to(struct tree *head, struct tree *remote, int reset)
+{
+	struct lock_file *lock_file;
+	struct unpack_trees_options opts;
+	struct tree_desc t[2];
+
+	if (parse_tree(head) || parse_tree(remote))
+		return -1;
+
+	lock_file = xcalloc(1, sizeof(struct lock_file));
+	hold_locked_index(lock_file, 1);
+
+	refresh_cache(REFRESH_QUIET);
+
+	memset(&opts, 0, sizeof(opts));
+	opts.head_idx = 1;
+	opts.src_index = &the_index;
+	opts.dst_index = &the_index;
+	opts.update = 1;
+	opts.merge = 1;
+	opts.reset = reset;
+	opts.fn = twoway_merge;
+	init_tree_desc(&t[0], head->buffer, head->size);
+	init_tree_desc(&t[1], remote->buffer, remote->size);
+
+	if (unpack_trees(2, t, &opts)) {
+		rollback_lock_file(lock_file);
+		return -1;
+	}
+
+	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+		die(_("unable to write new index file"));
+
+	return 0;
+}
+
+/**
+ * Clean the index without touching entries that are not modified between
+ * `head` and `remote`.
+ */
+static int clean_index(const unsigned char *head, const unsigned char *remote)
+{
+	struct lock_file *lock_file;
+	struct tree *head_tree, *remote_tree, *index_tree;
+	unsigned char index[GIT_SHA1_RAWSZ];
+	struct pathspec pathspec;
+
+	head_tree = parse_tree_indirect(head);
+	if (!head_tree)
+		return error(_("Could not parse object '%s'."), sha1_to_hex(head));
+
+	remote_tree = parse_tree_indirect(remote);
+	if (!remote_tree)
+		return error(_("Could not parse object '%s'."), sha1_to_hex(remote));
+
+	read_cache_unmerged();
+
+	if (fast_forward_to(head_tree, head_tree, 1))
+		return -1;
+
+	if (write_cache_as_tree(index, 0, NULL))
+		return -1;
+
+	index_tree = parse_tree_indirect(index);
+	if (!index_tree)
+		return error(_("Could not parse object '%s'."), sha1_to_hex(index));
+
+	if (fast_forward_to(index_tree, remote_tree, 0))
+		return -1;
+
+	memset(&pathspec, 0, sizeof(pathspec));
+
+	lock_file = xcalloc(1, sizeof(struct lock_file));
+	hold_locked_index(lock_file, 1);
+
+	if (read_tree(remote_tree, 0, &pathspec)) {
+		rollback_lock_file(lock_file);
+		return -1;
+	}
+
+	if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+		die(_("unable to write new index file"));
+
+	remove_branch_state();
+
+	return 0;
+}
+
+/**
+ * Resets rerere's merge resolution metadata.
+ */
+static void am_rerere_clear(void)
+{
+	struct string_list merge_rr = STRING_LIST_INIT_DUP;
+	int fd = setup_rerere(&merge_rr, 0);
+
+	if (fd < 0)
+		return;
+
+	rerere_clear(&merge_rr);
+	string_list_clear(&merge_rr, 1);
+}
+
+/**
+ * Resume the current am session by skipping the current patch.
+ */
+static void am_skip(struct am_state *state)
+{
+	unsigned char head[GIT_SHA1_RAWSZ];
+
+	am_rerere_clear();
+
+	if (get_sha1("HEAD", head))
+		hashcpy(head, EMPTY_TREE_SHA1_BIN);
+
+	if (clean_index(head, head))
+		die(_("failed to clean index"));
+
+	am_next(state);
+	am_run(state, 0);
+}
+
+/**
+ * Returns true if it is safe to reset HEAD to the ORIG_HEAD, false otherwise.
+ *
+ * It is not safe to reset HEAD when:
+ * 1. git-am previously failed because the index was dirty.
+ * 2. HEAD has moved since git-am previously failed.
+ */
+static int safe_to_abort(const struct am_state *state)
+{
+	struct strbuf sb = STRBUF_INIT;
+	unsigned char abort_safety[GIT_SHA1_RAWSZ], head[GIT_SHA1_RAWSZ];
+
+	if (file_exists(am_path(state, "dirtyindex")))
+		return 0;
+
+	if (read_state_file(&sb, state, "abort-safety", 1) > 0) {
+		if (get_sha1_hex(sb.buf, abort_safety))
+			die(_("could not parse %s"), am_path(state, "abort_safety"));
+	} else
+		hashclr(abort_safety);
+
+	if (get_sha1("HEAD", head))
+		hashclr(head);
+
+	if (!hashcmp(head, abort_safety))
+		return 1;
+
+	error(_("You seem to have moved HEAD since the last 'am' failure.\n"
+		"Not rewinding to ORIG_HEAD"));
+
+	return 0;
+}
+
+/**
+ * Aborts the current am session if it is safe to do so.
+ */
+static void am_abort(struct am_state *state)
+{
+	unsigned char curr_head[GIT_SHA1_RAWSZ], orig_head[GIT_SHA1_RAWSZ];
+	int has_curr_head, has_orig_head;
+	char *curr_branch;
+
+	if (!safe_to_abort(state)) {
+		am_destroy(state);
+		return;
+	}
+
+	am_rerere_clear();
+
+	curr_branch = resolve_refdup("HEAD", 0, curr_head, NULL);
+	has_curr_head = !is_null_sha1(curr_head);
+	if (!has_curr_head)
+		hashcpy(curr_head, EMPTY_TREE_SHA1_BIN);
+
+	has_orig_head = !get_sha1("ORIG_HEAD", orig_head);
+	if (!has_orig_head)
+		hashcpy(orig_head, EMPTY_TREE_SHA1_BIN);
+
+	clean_index(curr_head, orig_head);
+
+	if (has_orig_head)
+		update_ref("am --abort", "HEAD", orig_head,
+				has_curr_head ? curr_head : NULL, 0,
+				UPDATE_REFS_DIE_ON_ERR);
+	else if (curr_branch)
+		delete_ref(curr_branch, NULL, REF_NODEREF);
+
+	free(curr_branch);
+	am_destroy(state);
+}
+
+/**
+ * parse_options() callback that validates and sets opt->value to the
+ * PATCH_FORMAT_* enum value corresponding to `arg`.
+ */
+static int parse_opt_patchformat(const struct option *opt, const char *arg, int unset)
+{
+	int *opt_value = opt->value;
+
+	if (!strcmp(arg, "mbox"))
+		*opt_value = PATCH_FORMAT_MBOX;
+	else if (!strcmp(arg, "stgit"))
+		*opt_value = PATCH_FORMAT_STGIT;
+	else if (!strcmp(arg, "stgit-series"))
+		*opt_value = PATCH_FORMAT_STGIT_SERIES;
+	else if (!strcmp(arg, "hg"))
+		*opt_value = PATCH_FORMAT_HG;
+	else
+		return error(_("Invalid value for --patch-format: %s"), arg);
+	return 0;
+}
+
+enum resume_mode {
+	RESUME_FALSE = 0,
+	RESUME_APPLY,
+	RESUME_RESOLVED,
+	RESUME_SKIP,
+	RESUME_ABORT
+};
+
+int cmd_am(int argc, const char **argv, const char *prefix)
+{
+	struct am_state state;
+	int binary = -1;
+	int keep_cr = -1;
+	int patch_format = PATCH_FORMAT_UNKNOWN;
+	enum resume_mode resume = RESUME_FALSE;
+
+	const char * const usage[] = {
+		N_("git am [options] [(<mbox>|<Maildir>)...]"),
+		N_("git am [options] (--continue | --skip | --abort)"),
+		NULL
+	};
+
+	struct option options[] = {
+		OPT_BOOL('i', "interactive", &state.interactive,
+			N_("run interactively")),
+		OPT_HIDDEN_BOOL('b', "binary", &binary,
+			N_("(historical option -- no-op")),
+		OPT_BOOL('3', "3way", &state.threeway,
+			N_("allow fall back on 3way merging if needed")),
+		OPT__QUIET(&state.quiet, N_("be quiet")),
+		OPT_BOOL('s', "signoff", &state.signoff,
+			N_("add a Signed-off-by line to the commit message")),
+		OPT_BOOL('u', "utf8", &state.utf8,
+			N_("recode into utf8 (default)")),
+		OPT_SET_INT('k', "keep", &state.keep,
+			N_("pass -k flag to git-mailinfo"), KEEP_TRUE),
+		OPT_SET_INT(0, "keep-non-patch", &state.keep,
+			N_("pass -b flag to git-mailinfo"), KEEP_NON_PATCH),
+		OPT_BOOL('m', "message-id", &state.message_id,
+			N_("pass -m flag to git-mailinfo")),
+		{ OPTION_SET_INT, 0, "keep-cr", &keep_cr, NULL,
+		  N_("pass --keep-cr flag to git-mailsplit for mbox format"),
+		  PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, 1},
+		{ OPTION_SET_INT, 0, "no-keep-cr", &keep_cr, NULL,
+		  N_("do not pass --keep-cr flag to git-mailsplit independent of am.keepcr"),
+		  PARSE_OPT_NOARG | PARSE_OPT_NONEG, NULL, 0},
+		OPT_BOOL('c', "scissors", &state.scissors,
+			N_("strip everything before a scissors line")),
+		OPT_PASSTHRU_ARGV(0, "whitespace", &state.git_apply_opts, N_("action"),
+			N_("pass it through git-apply"),
+			0),
+		OPT_PASSTHRU_ARGV(0, "ignore-space-change", &state.git_apply_opts, NULL,
+			N_("pass it through git-apply"),
+			PARSE_OPT_NOARG),
+		OPT_PASSTHRU_ARGV(0, "ignore-whitespace", &state.git_apply_opts, NULL,
+			N_("pass it through git-apply"),
+			PARSE_OPT_NOARG),
+		OPT_PASSTHRU_ARGV(0, "directory", &state.git_apply_opts, N_("root"),
+			N_("pass it through git-apply"),
+			0),
+		OPT_PASSTHRU_ARGV(0, "exclude", &state.git_apply_opts, N_("path"),
+			N_("pass it through git-apply"),
+			0),
+		OPT_PASSTHRU_ARGV(0, "include", &state.git_apply_opts, N_("path"),
+			N_("pass it through git-apply"),
+			0),
+		OPT_PASSTHRU_ARGV('C', NULL, &state.git_apply_opts, N_("n"),
+			N_("pass it through git-apply"),
+			0),
+		OPT_PASSTHRU_ARGV('p', NULL, &state.git_apply_opts, N_("num"),
+			N_("pass it through git-apply"),
+			0),
+		OPT_CALLBACK(0, "patch-format", &patch_format, N_("format"),
+			N_("format the patch(es) are in"),
+			parse_opt_patchformat),
+		OPT_PASSTHRU_ARGV(0, "reject", &state.git_apply_opts, NULL,
+			N_("pass it through git-apply"),
+			PARSE_OPT_NOARG),
+		OPT_STRING(0, "resolvemsg", &state.resolvemsg, NULL,
+			N_("override error message when patch failure occurs")),
+		OPT_CMDMODE(0, "continue", &resume,
+			N_("continue applying patches after resolving a conflict"),
+			RESUME_RESOLVED),
+		OPT_CMDMODE('r', "resolved", &resume,
+			N_("synonyms for --continue"),
+			RESUME_RESOLVED),
+		OPT_CMDMODE(0, "skip", &resume,
+			N_("skip the current patch"),
+			RESUME_SKIP),
+		OPT_CMDMODE(0, "abort", &resume,
+			N_("restore the original branch and abort the patching operation."),
+			RESUME_ABORT),
+		OPT_BOOL(0, "committer-date-is-author-date",
+			&state.committer_date_is_author_date,
+			N_("lie about committer date")),
+		OPT_BOOL(0, "ignore-date", &state.ignore_date,
+			N_("use current timestamp for author date")),
+		OPT_RERERE_AUTOUPDATE(&state.allow_rerere_autoupdate),
+		{ OPTION_STRING, 'S', "gpg-sign", &state.sign_commit, N_("key-id"),
+		  N_("GPG-sign commits"),
+		  PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
+		OPT_HIDDEN_BOOL(0, "rebasing", &state.rebasing,
+			N_("(internal use for git-rebase)")),
+		OPT_END()
+	};
+
+	git_config(git_default_config, NULL);
+
+	am_state_init(&state, git_path("rebase-apply"));
+
+	argc = parse_options(argc, argv, prefix, options, usage, 0);
+
+	if (binary >= 0)
+		fprintf_ln(stderr, _("The -b/--binary option has been a no-op for long time, and\n"
+				"it will be removed. Please do not use it anymore."));
+
+	/* Ensure a valid committer ident can be constructed */
+	git_committer_info(IDENT_STRICT);
+
+	if (read_index_preload(&the_index, NULL) < 0)
+		die(_("failed to read the index"));
+
+	if (am_in_progress(&state)) {
+		/*
+		 * Catch user error to feed us patches when there is a session
+		 * in progress:
+		 *
+		 * 1. mbox path(s) are provided on the command-line.
+		 * 2. stdin is not a tty: the user is trying to feed us a patch
+		 *    from standard input. This is somewhat unreliable -- stdin
+		 *    could be /dev/null for example and the caller did not
+		 *    intend to feed us a patch but wanted to continue
+		 *    unattended.
+		 */
+		if (argc || (resume == RESUME_FALSE && !isatty(0)))
+			die(_("previous rebase directory %s still exists but mbox given."),
+				state.dir);
+
+		if (resume == RESUME_FALSE)
+			resume = RESUME_APPLY;
+
+		am_load(&state);
+	} else {
+		struct argv_array paths = ARGV_ARRAY_INIT;
+		int i;
+
+		/*
+		 * Handle stray state directory in the independent-run case. In
+		 * the --rebasing case, it is up to the caller to take care of
+		 * stray directories.
+		 */
+		if (file_exists(state.dir) && !state.rebasing) {
+			if (resume == RESUME_ABORT) {
+				am_destroy(&state);
+				am_state_release(&state);
+				return 0;
+			}
+
+			die(_("Stray %s directory found.\n"
+				"Use \"git am --abort\" to remove it."),
+				state.dir);
+		}
+
+		if (resume)
+			die(_("Resolve operation not in progress, we are not resuming."));
+
+		for (i = 0; i < argc; i++) {
+			if (is_absolute_path(argv[i]) || !prefix)
+				argv_array_push(&paths, argv[i]);
+			else
+				argv_array_push(&paths, mkpath("%s/%s", prefix, argv[i]));
+		}
+
+		am_setup(&state, patch_format, paths.argv, keep_cr);
+
+		argv_array_clear(&paths);
+	}
+
+	switch (resume) {
+	case RESUME_FALSE:
+		am_run(&state, 0);
+		break;
+	case RESUME_APPLY:
+		am_run(&state, 1);
+		break;
+	case RESUME_RESOLVED:
+		am_resolve(&state);
+		break;
+	case RESUME_SKIP:
+		am_skip(&state);
+		break;
+	case RESUME_ABORT:
+		am_abort(&state);
+		break;
+	default:
+		die("BUG: invalid resume value");
+	}
+
+	am_state_release(&state);
+
+	return 0;
+}
diff --git a/cache-tree.c b/cache-tree.c
index 32772b9564..feace8bd90 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -592,7 +592,7 @@ static struct cache_tree *cache_tree_find(struct cache_tree *it, const char *pat
 	return it;
 }
 
-int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix)
+int write_index_as_tree(unsigned char *sha1, struct index_state *index_state, const char *index_path, int flags, const char *prefix)
 {
 	int entries, was_valid, newfd;
 	struct lock_file *lock_file;
@@ -603,23 +603,23 @@ int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix)
 	 */
 	lock_file = xcalloc(1, sizeof(struct lock_file));
 
-	newfd = hold_locked_index(lock_file, 1);
+	newfd = hold_lock_file_for_update(lock_file, index_path, LOCK_DIE_ON_ERROR);
 
-	entries = read_cache();
+	entries = read_index_from(index_state, index_path);
 	if (entries < 0)
 		return WRITE_TREE_UNREADABLE_INDEX;
 	if (flags & WRITE_TREE_IGNORE_CACHE_TREE)
-		cache_tree_free(&(active_cache_tree));
+		cache_tree_free(&index_state->cache_tree);
 
-	if (!active_cache_tree)
-		active_cache_tree = cache_tree();
+	if (!index_state->cache_tree)
+		index_state->cache_tree = cache_tree();
 
-	was_valid = cache_tree_fully_valid(active_cache_tree);
+	was_valid = cache_tree_fully_valid(index_state->cache_tree);
 	if (!was_valid) {
-		if (cache_tree_update(&the_index, flags) < 0)
+		if (cache_tree_update(index_state, flags) < 0)
 			return WRITE_TREE_UNMERGED_INDEX;
 		if (0 <= newfd) {
-			if (!write_locked_index(&the_index, lock_file, COMMIT_LOCK))
+			if (!write_locked_index(index_state, lock_file, COMMIT_LOCK))
 				newfd = -1;
 		}
 		/* Not being able to write is fine -- we are only interested
@@ -631,14 +631,14 @@ int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix)
 	}
 
 	if (prefix) {
-		struct cache_tree *subtree =
-			cache_tree_find(active_cache_tree, prefix);
+		struct cache_tree *subtree;
+		subtree = cache_tree_find(index_state->cache_tree, prefix);
 		if (!subtree)
 			return WRITE_TREE_PREFIX_ERROR;
 		hashcpy(sha1, subtree->sha1);
 	}
 	else
-		hashcpy(sha1, active_cache_tree->sha1);
+		hashcpy(sha1, index_state->cache_tree->sha1);
 
 	if (0 <= newfd)
 		rollback_lock_file(lock_file);
@@ -646,6 +646,11 @@ int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix)
 	return 0;
 }
 
+int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix)
+{
+	return write_index_as_tree(sha1, &the_index, get_index_file(), flags, prefix);
+}
+
 static void prime_cache_tree_rec(struct cache_tree *it, struct tree *tree)
 {
 	struct tree_desc desc;
diff --git a/cache-tree.h b/cache-tree.h
index aa7b3e4a0a..41c574663a 100644
--- a/cache-tree.h
+++ b/cache-tree.h
@@ -46,6 +46,7 @@ int update_main_cache_tree(int);
 #define WRITE_TREE_UNMERGED_INDEX (-2)
 #define WRITE_TREE_PREFIX_ERROR (-3)
 
+int write_index_as_tree(unsigned char *sha1, struct index_state *index_state, const char *index_path, int flags, const char *prefix);
 int write_cache_as_tree(unsigned char *sha1, int flags, const char *prefix);
 void prime_cache_tree(struct index_state *, struct tree *);
 
diff --git a/git-am.sh b/contrib/examples/git-am.sh
similarity index 100%
rename from git-am.sh
rename to contrib/examples/git-am.sh
diff --git a/git-compat-util.h b/git-compat-util.h
index c6d391f864..392da79029 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -717,10 +717,12 @@ extern void *xrealloc(void *ptr, size_t size);
 extern void *xcalloc(size_t nmemb, size_t size);
 extern void *xmmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);
 extern void *xmmap_gently(void *start, size_t length, int prot, int flags, int fd, off_t offset);
+extern int xopen(const char *path, int flags, ...);
 extern ssize_t xread(int fd, void *buf, size_t len);
 extern ssize_t xwrite(int fd, const void *buf, size_t len);
 extern ssize_t xpread(int fd, void *buf, size_t len, off_t offset);
 extern int xdup(int fd);
+extern FILE *xfopen(const char *path, const char *mode);
 extern FILE *xfdopen(int fd, const char *mode);
 extern int xmkstemp(char *template);
 extern int xmkstemp_mode(char *template, int mode);
diff --git a/git.c b/git.c
index 55c327c7b3..5feba410ca 100644
--- a/git.c
+++ b/git.c
@@ -370,6 +370,7 @@ static int run_builtin(struct cmd_struct *p, int argc, const char **argv)
 
 static struct cmd_struct commands[] = {
 	{ "add", cmd_add, RUN_SETUP | NEED_WORK_TREE },
+	{ "am", cmd_am, RUN_SETUP | NEED_WORK_TREE },
 	{ "annotate", cmd_annotate, RUN_SETUP },
 	{ "apply", cmd_apply, RUN_SETUP_GENTLY },
 	{ "archive", cmd_archive },
diff --git a/t/t4150-am.sh b/t/t4150-am.sh
index e9b6f8158a..dd627c42d3 100755
--- a/t/t4150-am.sh
+++ b/t/t4150-am.sh
@@ -551,6 +551,25 @@ test_expect_success 'am -3 -p0 can read --no-prefix patch' '
 	git diff --exit-code lorem
 '
 
+test_expect_success 'am with config am.threeWay falls back to 3-way merge' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout -b lorem4 base3way &&
+	test_config am.threeWay 1 &&
+	git am lorem-move.patch &&
+	test_path_is_missing .git/rebase-apply &&
+	git diff --exit-code lorem
+'
+
+test_expect_success 'am with config am.threeWay overridden by --no-3way' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout -b lorem5 base3way &&
+	test_config am.threeWay 1 &&
+	test_must_fail git am --no-3way lorem-move.patch &&
+	test_path_is_dir .git/rebase-apply
+'
+
 test_expect_success 'am can rename a file' '
 	grep "^rename from" rename.patch &&
 	rm -fr .git/rebase-apply &&
diff --git a/wrapper.c b/wrapper.c
index ff49807948..e451463431 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -189,6 +189,41 @@ void *xcalloc(size_t nmemb, size_t size)
 # endif
 #endif
 
+/**
+ * xopen() is the same as open(), but it die()s if the open() fails.
+ */
+int xopen(const char *path, int oflag, ...)
+{
+	mode_t mode = 0;
+	va_list ap;
+
+	/*
+	 * va_arg() will have undefined behavior if the specified type is not
+	 * compatible with the argument type. Since integers are promoted to
+	 * ints, we fetch the next argument as an int, and then cast it to a
+	 * mode_t to avoid undefined behavior.
+	 */
+	va_start(ap, oflag);
+	if (oflag & O_CREAT)
+		mode = va_arg(ap, int);
+	va_end(ap);
+
+	for (;;) {
+		int fd = open(path, oflag, mode);
+		if (fd >= 0)
+			return fd;
+		if (errno == EINTR)
+			continue;
+
+		if ((oflag & O_RDWR) == O_RDWR)
+			die_errno(_("could not open '%s' for reading and writing"), path);
+		else if ((oflag & O_WRONLY) == O_WRONLY)
+			die_errno(_("could not open '%s' for writing"), path);
+		else
+			die_errno(_("could not open '%s' for reading"), path);
+	}
+}
+
 /*
  * xread() is the same a read(), but it automatically restarts read()
  * operations with a recoverable error (EAGAIN and EINTR). xread()
@@ -311,6 +346,27 @@ int xdup(int fd)
 	return ret;
 }
 
+/**
+ * xfopen() is the same as fopen(), but it die()s if the fopen() fails.
+ */
+FILE *xfopen(const char *path, const char *mode)
+{
+	for (;;) {
+		FILE *fp = fopen(path, mode);
+		if (fp)
+			return fp;
+		if (errno == EINTR)
+			continue;
+
+		if (*mode && mode[1] == '+')
+			die_errno(_("could not open '%s' for reading and writing"), path);
+		else if (*mode == 'w' || *mode == 'a')
+			die_errno(_("could not open '%s' for writing"), path);
+		else
+			die_errno(_("could not open '%s' for reading"), path);
+	}
+}
+
 FILE *xfdopen(int fd, const char *mode)
 {
 	FILE *stream = fdopen(fd, mode);