fmt-merge-msg: be quiet if nothing to merge

When FETCH_HEAD contains only 'not-for-merge' entries fmt-merge-msg
still outputs "Merge" (and if the branch isn't master " into <branch>").
In this case fmt-merge-msg is outputting junk and should really just
be quiet. Fix it.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fmt-merge-msg.c

@@ -281,6 +281,9 @@ int fmt_merge_msg(int merge_summary, struct strbuf *in, struct strbuf *out) {
 			die ("Error in line %d: %.*s", i, len, p);
 	}
 
+	if (!srcs.nr)
+		return 0;
+
 	strbuf_addstr(out, "Merge ");
 	for (i = 0; i < srcs.nr; i++) {
 		struct src_data *src_data = srcs.payload[i];

t/t6200-fmt-merge-msg.sh

@@ -21,6 +21,8 @@ test_expect_success setup '
 	setdate &&
 	git commit -m "Initial" &&
 
+	git clone . remote &&
+
 	echo uno >one &&
 	echo dos >two &&
 	git add two &&
@@ -240,4 +242,21 @@ test_expect_success 'merge-msg -F in subdirectory' '
 	test_cmp expected actual
 '
 
+test_expect_success 'merge-msg with nothing to merge' '
+
+	git config --unset-all merge.log
+	git config --unset-all merge.summary
+	git config merge.summary yes &&
+
+	(
+		cd remote &&
+		git checkout -b unrelated &&
+		setdate &&
+		git fetch origin &&
+		git fmt-merge-msg <.git/FETCH_HEAD >../actual
+	) &&
+
+	test_cmp /dev/null actual
+'
+
 test_done