fmt-merge-msg -m to override merge title

Since v1.7.1.1~23^2 (merge: --log appends shortlog to message if
specified, 2010-05-11), the fmt-merge-msg backend supports custom text
to override the merge title "Merge <foo> into <bar>".

Expose this functionality for scripted callers.  Example:

 git fmt-merge-msg --log -m \
	"$(printf '%s\n' \
	    "Merge branch 'api-cleanup' into feature" \
	    '' \
	    'This is to use a few functions refactored for this purpose.'
	)" <.git/FETCH_HEAD

Cc: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fmt-merge-msg.txt

@@ -9,8 +9,8 @@ git-fmt-merge-msg - Produce a merge commit message
 SYNOPSIS
 --------
 [verse]
-'git fmt-merge-msg' [--log | --no-log] <$GIT_DIR/FETCH_HEAD
-'git fmt-merge-msg' [--log | --no-log] -F <file>
+'git fmt-merge-msg' [-m <message>] [--log | --no-log] <$GIT_DIR/FETCH_HEAD
+'git fmt-merge-msg' [-m <message>] [--log | --no-log] -F <file>
 
 DESCRIPTION
 -----------
@@ -38,6 +38,11 @@ OPTIONS
 	Synonyms to --log and --no-log; these are deprecated and will be
 	removed in the future.
 
+-m <message>::
+--message <message>::
+	Use <message> instead of the branch names for the first line
+	of the log message.  For use with `--log`.
+
 -F <file>::
 --file <file>::
 	Take the list of merged objects from <file> instead of

builtin/fmt-merge-msg.c

@@ -7,7 +7,7 @@
 #include "string-list.h"
 
 static const char * const fmt_merge_msg_usage[] = {
-	"git fmt-merge-msg [--log|--no-log] [--file <file>]",
+	"git fmt-merge-msg [-m <message>] [--log|--no-log] [--file <file>]",
 	NULL
 };
 
@@ -319,11 +319,14 @@ int fmt_merge_msg_shortlog(struct strbuf *in, struct strbuf *out) {
 int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 {
 	const char *inpath = NULL;
+	const char *message = NULL;
 	struct option options[] = {
 		OPT_BOOLEAN(0, "log",     &merge_summary, "populate log with the shortlog"),
 		{ OPTION_BOOLEAN, 0, "summary", &merge_summary, NULL,
 		  "alias for --log (deprecated)",
 		  PARSE_OPT_NOARG | PARSE_OPT_HIDDEN },
+		OPT_STRING('m', "message", &message, "text",
+			"use <text> as start of message"),
 		OPT_FILENAME('F', "file", &inpath, "file to read from"),
 		OPT_END()
 	};
@@ -337,6 +340,12 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 			     0);
 	if (argc > 0)
 		usage_with_options(fmt_merge_msg_usage, options);
+	if (message && !merge_summary) {
+		char nl = '\n';
+		write_in_full(STDOUT_FILENO, message, strlen(message));
+		write_in_full(STDOUT_FILENO, &nl, 1);
+		return 0;
+	}
 
 	if (inpath && strcmp(inpath, "-")) {
 		in = fopen(inpath, "r");
@@ -346,7 +355,12 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 
 	if (strbuf_read(&input, fileno(in), 0) < 0)
 		die_errno("could not read input file");
-	ret = fmt_merge_msg(merge_summary, &input, &output);
+	if (message) {
+		strbuf_addstr(&output, message);
+		ret = fmt_merge_msg_shortlog(&input, &output);
+	} else {
+		ret = fmt_merge_msg(merge_summary, &input, &output);
+	}
 	if (ret)
 		return ret;
 	write_in_full(STDOUT_FILENO, output.buf, output.len);

t/t6200-fmt-merge-msg.sh

@@ -129,6 +129,38 @@ test_expect_success '[merge] summary/log configuration' '
 	test_cmp expected actual2
 '
 
+test_expect_success 'fmt-merge-msg -m' '
+	echo "Sync with left" >expected &&
+	cat >expected.log <<-EOF &&
+	Sync with left
+
+	* ${apos}left${apos} of $(pwd):
+	  Left #5
+	  Left #4
+	  Left #3
+	  Common #2
+	  Common #1
+	EOF
+
+	test_might_fail git config --unset merge.log &&
+	test_might_fail git config --unset merge.summary &&
+	git checkout master &&
+	git fetch "$(pwd)" left &&
+	git fmt-merge-msg -m "Sync with left" <.git/FETCH_HEAD >actual &&
+	git fmt-merge-msg --log -m "Sync with left" \
+					<.git/FETCH_HEAD >actual.log &&
+	git config merge.log true &&
+	git fmt-merge-msg -m "Sync with left" \
+					<.git/FETCH_HEAD >actual.log-config &&
+	git fmt-merge-msg --no-log -m "Sync with left" \
+					<.git/FETCH_HEAD >actual.nolog &&
+
+	test_cmp expected actual &&
+	test_cmp expected.log actual.log &&
+	test_cmp expected.log actual.log-config &&
+	test_cmp expected actual.nolog
+'
+
 test_expect_success 'setup: expected shortlog for two branches' '
 	cat >expected <<-EOF
 	Merge branches ${apos}left${apos} and ${apos}right${apos}