format-patch: allow --rfc to optionally take a value, like --rfc=WIP

With the "--rfc" option, we can tweak the "[PATCH]" (or whatever
string specified with the "--subject-prefix" option, instead of
"PATCH") that we prefix the title of the commit with into "[RFC
PATCH]", but some projects may want "[rfc PATCH]".  Adding a new
option, e.g., "--rfc-lowercase", to support such need every time
somebody wants to use different strings would lead to insanity of
accumulating unbounded number of such options.

Allow an optional value specified for the option, so that users can
use "--rfc=rfc" (think of "--rfc" without value as a short-hand for
"--rfc=RFC") if they wanted to.

This can of course be (ab)used to make the prefix "[WIP PATCH]" by
passing "--rfc=WIP".  Passing an empty string, i.e., "--rfc=", is
the same as "--no-rfc" to override an option given earlier on the
same command line.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -20,7 +20,7 @@ SYNOPSIS
 		   [--in-reply-to=<message-id>] [--suffix=.<sfx>]
 		   [--ignore-if-in-upstream] [--always]
 		   [--cover-from-description=<mode>]
-		   [--rfc] [--subject-prefix=<subject-prefix>]
+		   [--rfc[=<rfc>]] [--subject-prefix=<subject-prefix>]
 		   [(--reroll-count|-v) <n>]
 		   [--to=<email>] [--cc=<email>]
 		   [--[no-]cover-letter] [--quiet]
@@ -238,10 +238,15 @@ the patches (with a value of e.g. "PATCH my-project").
 	value of the `format.filenameMaxLength` configuration
 	variable, or 64 if unconfigured.
 
---rfc::
-	Prepends "RFC" to the subject prefix (producing "RFC PATCH" by
-	default). RFC means "Request For Comments"; use this when sending
-	an experimental patch for discussion rather than application.
+--rfc[=<rfc>]::
+	Prepends the string _<rfc>_ (defaults to "RFC") to
+	the subject prefix.  As the subject prefix defaults to
+	"PATCH", you'll get "RFC PATCH" by default.
++
+RFC means "Request For Comments"; use this when sending
+an experimental patch for discussion rather than application.
+"--rfc=WIP" may also be a useful way to indicate that a patch
+is not complete yet ("WIP" stands for "Work In Progress").
 
 -v <n>::
 --reroll-count=<n>::

builtin/log.c

@@ -1494,6 +1494,19 @@ static int subject_prefix_callback(const struct option *opt, const char *arg,
 	return 0;
 }
 
+static int rfc_callback(const struct option *opt, const char *arg,
+			int unset)
+{
+	const char **rfc = opt->value;
+
+	*rfc = opt->value;
+	if (unset)
+		*rfc = NULL;
+	else
+		*rfc = arg ? arg : "RFC";
+	return 0;
+}
+
 static int numbered_cmdline_opt = 0;
 
 static int numbered_callback(const struct option *opt, const char *arg,
@@ -1907,8 +1920,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	struct strbuf rdiff2 = STRBUF_INIT;
 	struct strbuf rdiff_title = STRBUF_INIT;
 	struct strbuf sprefix = STRBUF_INIT;
+	const char *rfc = NULL;
 	int creation_factor = -1;
-	int rfc = 0;
 
 	const struct option builtin_format_patch_options[] = {
 		OPT_CALLBACK_F('n', "numbered", &numbered, NULL,
@@ -1932,7 +1945,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			    N_("mark the series as Nth re-roll")),
 		OPT_INTEGER(0, "filename-max-length", &fmt_patch_name_max,
 			    N_("max length of output filename")),
-		OPT_BOOL(0, "rfc", &rfc, N_("use [RFC PATCH] instead of [PATCH]")),
+		OPT_CALLBACK_F(0, "rfc", &rfc, N_("rfc"),
+			       N_("add <rfc> (default 'RFC') before 'PATCH'"),
+			       PARSE_OPT_OPTARG, rfc_callback),
 		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")),
@@ -2050,8 +2065,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	if (cover_from_description_arg)
 		cover_from_description_mode = parse_cover_from_description(cover_from_description_arg);
 
-	if (rfc)
-		strbuf_insertstr(&sprefix, 0, "RFC ");
+	if (rfc && rfc[0])
+		strbuf_insertf(&sprefix, 0, "%s ", rfc);
 
 	if (reroll_count) {
 		strbuf_addf(&sprefix, " v%s", reroll_count);

t/t4014-format-patch.sh

@@ -1368,13 +1368,30 @@ test_expect_success 'empty subject prefix does not have extra space' '
 	test_cmp expect actual
 '
 
-test_expect_success '--rfc' '
+test_expect_success '--rfc and --no-rfc' '
 	cat >expect <<-\EOF &&
 	Subject: [RFC PATCH 1/1] header with . in it
 	EOF
 	git format-patch -n -1 --stdout --rfc >patch &&
 	grep "^Subject:" patch >actual &&
-	test_cmp expect actual
+	test_cmp expect actual &&
+	git format-patch -n -1 --stdout --rfc --no-rfc >patch &&
+	sed -e "s/RFC //" expect >expect-raw &&
+	grep "^Subject:" patch >actual &&
+	test_cmp expect-raw actual
+'
+
+test_expect_success '--rfc=WIP and --rfc=' '
+	cat >expect <<-\EOF &&
+	Subject: [WIP PATCH 1/1] header with . in it
+	EOF
+	git format-patch -n -1 --stdout --rfc=WIP >patch &&
+	grep "^Subject:" patch >actual &&
+	test_cmp expect actual &&
+	git format-patch -n -1 --stdout --rfc --rfc= >patch &&
+	sed -e "s/WIP //" expect >expect-raw &&
+	grep "^Subject:" patch >actual &&
+	test_cmp expect-raw actual
 '
 
 test_expect_success '--rfc does not overwrite prefix' '