format-patch: add "--rfc" for the common case of [RFC PATCH]

Add an alias for --subject-prefix='RFC PATCH', which is used
commonly in some development communities to deserve such a
short-hand.
Signed-off-by: NJosh Triplett <josh@joshtriplett.org>
Reviewed-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -19,7 +19,8 @@ SYNOPSIS
 		   [--start-number <n>] [--numbered-files]
 		   [--in-reply-to=Message-Id] [--suffix=.<sfx>]
 		   [--ignore-if-in-upstream]
-		   [--subject-prefix=Subject-Prefix] [(--reroll-count|-v) <n>]
+		   [--rfc] [--subject-prefix=Subject-Prefix]
+		   [(--reroll-count|-v) <n>]
 		   [--to=<email>] [--cc=<email>]
 		   [--[no-]cover-letter] [--quiet] [--notes[=<ref>]]
 		   [<common diff options>]
@@ -172,6 +173,11 @@ will want to ensure that threading is disabled for `git send-email`.
 	allows for useful naming of a patch series, and can be
 	combined with the `--numbered` option.
 
+--rfc::
+	Alias for `--subject-prefix="RFC PATCH"`. RFC means "Request For
+	Comments"; use this when sending an experimental patch for
+	discussion rather than application.
+
 -v <n>::
 --reroll-count=<n>::
 	Mark the series as the <n>-th iteration of the topic. The

builtin/log.c

@@ -1112,6 +1112,11 @@ 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)
+{
+	return subject_prefix_callback(opt, "RFC PATCH", unset);
+}
+
 static int numbered_cmdline_opt = 0;
 
 static int numbered_callback(const struct option *opt, const char *arg,
@@ -1419,6 +1424,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			    N_("start numbering patches at <n> instead of 1")),
 		OPT_INTEGER('v', "reroll-count", &reroll_count,
 			    N_("mark the series as Nth re-roll")),
+		{ OPTION_CALLBACK, 0, "rfc", &rev, NULL,
+			    N_("Use [RFC PATCH] instead of [PATCH]"),
+			    PARSE_OPT_NOARG | PARSE_OPT_NONEG, rfc_callback },
 		{ OPTION_CALLBACK, 0, "subject-prefix", &rev, N_("prefix"),
 			    N_("Use [<prefix>] instead of [PATCH]"),
 			    PARSE_OPT_NONEG, subject_prefix_callback },
@@ -1557,7 +1565,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	if (numbered && keep_subject)
 		die (_("-n and -k are mutually exclusive."));
 	if (keep_subject && subject_prefix)
-		die (_("--subject-prefix and -k are mutually exclusive."));
+		die (_("--subject-prefix/--rfc and -k are mutually exclusive."));
 	rev.preserve_subject = keep_subject;
 
 	argc = setup_revisions(argc, argv, &rev, &s_r_opt);

t/t4014-format-patch.sh

@@ -1073,6 +1073,15 @@ test_expect_success 'empty subject prefix does not have extra space' '
 	test_cmp expect actual
 '
 
+test_expect_success '--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_expect_success '--from=ident notices bogus ident' '
 	test_must_fail git format-patch -1 --stdout --from=foo >patch
 '