merge: do not add standard message when message is given with -m option

Even if the user explicitly gave her own message to "git merge", the
command still added its standard merge message.  It resulted in a
useless repetition like this:

    % git merge -m "Merge early part of side branch" `git rev-parse side~2`
    % git show -s
    commit 37217141e7519629353738d5e4e677a15096206f
    Merge: e68e646 a1d2374
    Author: しらいし ななこ <nanako3@lavabit.com>
    Date:   Wed Dec 2 14:33:20 2009 +0900

	Merge early part of side branch

	Merge commit 'a1d2374f8f52f4e8a53171601a920b538a6cec23'

The gave her own message because she didn't want git to add the
standard message (if she wanted to, she wouldn't have given one,
or she would have prepared it using git-fmt-merge-msg command).

Noticed by Nanako Shiraishi

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

builtin-merge.c

@@ -70,7 +70,7 @@ static int option_parse_message(const struct option *opt,
 	if (unset)
 		strbuf_setlen(buf, 0);
 	else if (arg) {
-		strbuf_addf(buf, "%s\n\n", arg);
+		strbuf_addf(buf, "%s%s", buf->len ? "\n\n" : "", arg);
 		have_message = 1;
 	} else
 		return error("switch `m' requires a value");
@@ -927,11 +927,13 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		 * codepath so we discard the error in this
 		 * loop.
 		 */
-		for (i = 0; i < argc; i++)
-			merge_name(argv[i], &msg);
-		fmt_merge_msg(option_log, &msg, &merge_msg);
-		if (merge_msg.len)
-			strbuf_setlen(&merge_msg, merge_msg.len-1);
+		if (!have_message) {
+			for (i = 0; i < argc; i++)
+				merge_name(argv[i], &msg);
+			fmt_merge_msg(option_log, &msg, &merge_msg);
+			if (merge_msg.len)
+				strbuf_setlen(&merge_msg, merge_msg.len-1);
+		}
 	}
 
 	if (head_invalid || !argc)

t/t7604-merge-custom-message.sh

@@ -22,15 +22,12 @@ test_expect_success 'setup' '
 	git tag c2
 '
 
-cat >expected <<\EOF
-custom message
 
-Merge commit 'c2'
-EOF
 test_expect_success 'merge c2 with a custom message' '
 	git reset --hard c1 &&
+	echo >expected "custom message" &&
 	git merge -m "custom message" c2 &&
-	git cat-file commit HEAD | sed -e "1,/^$/d" > actual &&
+	git cat-file commit HEAD | sed -e "1,/^$/d" >actual &&
 	test_cmp expected actual
 '