Merge branch 'ob/format-patch-description-file'

"git format-patch" learns a way to feed cover letter description,
that (1) can be used on detached HEAD where there is no branch
description available, and (2) also can override the branch
description if there is one.

* ob/format-patch-description-file:
  format-patch: add --description-file option

Documentation/git-format-patch.txt

@@ -215,6 +215,10 @@ is greater than 100 bytes, then the mode will be `message`, otherwise
 If `<mode>` is `none`, both the cover letter subject and body will be
 populated with placeholder text.
 
+--description-file=<file>::
+	Use the contents of <file> instead of the branch's description
+	for generating the cover letter.
+
 --subject-prefix=<subject prefix>::
 	Instead of the standard '[PATCH]' prefix in the subject
 	line, instead use '[<subject prefix>]'. This

builtin/log.c

@@ -1253,7 +1253,15 @@ static void show_diffstat(struct rev_info *rev,
 	fprintf(rev->diffopt.file, "\n");
 }
 
+static void read_desc_file(struct strbuf *buf, const char *desc_file)
+{
+	if (strbuf_read_file(buf, desc_file, 0) < 0)
+		die_errno(_("unable to read branch description file '%s'"),
+			  desc_file);
+}
+
 static void prepare_cover_text(struct pretty_print_context *pp,
+			       const char *description_file,
 			       const char *branch_name,
 			       struct strbuf *sb,
 			       const char *encoding,
@@ -1267,7 +1275,9 @@ static void prepare_cover_text(struct pretty_print_context *pp,
 	if (cover_from_description_mode == COVER_FROM_NONE)
 		goto do_pp;
 
-	if (branch_name && *branch_name)
+	if (description_file && *description_file)
+		read_desc_file(&description_sb, description_file);
+	else if (branch_name && *branch_name)
 		read_branch_desc(&description_sb, branch_name);
 	if (!description_sb.len)
 		goto do_pp;
@@ -1313,6 +1323,7 @@ static void get_notes_args(struct strvec *arg, struct rev_info *rev)
 static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 			      struct commit *origin,
 			      int nr, struct commit **list,
+			      const char *description_file,
 			      const char *branch_name,
 			      int quiet)
 {
@@ -1352,7 +1363,8 @@ static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 	pp.rev = rev;
 	pp.print_email_subject = 1;
 	pp_user_info(&pp, NULL, &sb, committer, encoding);
-	prepare_cover_text(&pp, branch_name, &sb, encoding, need_8bit_cte);
+	prepare_cover_text(&pp, description_file, branch_name, &sb,
+			   encoding, need_8bit_cte);
 	fprintf(rev->diffopt.file, "%s\n", sb.buf);
 
 	strbuf_release(&sb);
@@ -1893,6 +1905,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	int quiet = 0;
 	const char *reroll_count = NULL;
 	char *cover_from_description_arg = NULL;
+	char *description_file = NULL;
 	char *branch_name = NULL;
 	char *base_commit = NULL;
 	struct base_tree_info bases;
@@ -1936,6 +1949,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		OPT_STRING(0, "cover-from-description", &cover_from_description_arg,
 			    N_("cover-from-description-mode"),
 			    N_("generate parts of a cover letter based on a branch's description")),
+		OPT_FILENAME(0, "description-file", &description_file,
+			     N_("use branch description from file")),
 		OPT_CALLBACK_F(0, "subject-prefix", &rev, N_("prefix"),
 			    N_("use [<prefix>] instead of [PATCH]"),
 			    PARSE_OPT_NONEG, subject_prefix_callback),
@@ -2321,7 +2336,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		if (thread)
 			gen_message_id(&rev, "cover");
 		make_cover_letter(&rev, !!output_directory,
-				  origin, nr, list, branch_name, quiet);
+				  origin, nr, list, description_file, branch_name, quiet);
 		print_bases(&bases, rev.diffopt.file);
 		print_signature(rev.diffopt.file);
 		total++;

t/t4014-format-patch.sh

@@ -1991,6 +1991,20 @@ test_expect_success 'cover letter using branch description (6)' '
 	grep hello actual
 '
 
+test_expect_success 'cover letter with --description-file' '
+	test_when_finished "rm -f description.txt" &&
+	cat >description.txt <<-\EOF &&
+	subject from file
+
+	body from file
+	EOF
+	git checkout rebuild-1 &&
+	git format-patch --stdout --cover-letter --cover-from-description auto \
+		--description-file description.txt main >actual &&
+	grep "^Subject: \[PATCH 0/2\] subject from file$" actual &&
+	grep "^body from file$" actual
+'
+
 test_expect_success 'cover letter with nothing' '
 	git format-patch --stdout --cover-letter >actual &&
 	test_line_count = 0 actual