builtin-merge.c: Fix option parsing

Now "git merge -m" needs a message, and errors out with the usage
text if none is given.
This way, t7600-merge.sh is fixed.

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Acked-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge.c

@@ -66,10 +66,11 @@ static int option_parse_message(const struct option *opt,
 
 	if (unset)
 		strbuf_setlen(buf, 0);
-	else {
+	else if (arg) {
 		strbuf_addf(buf, "%s\n\n", arg);
 		have_message = 1;
-	}
+	} else
+		return error("switch `m' requires a value");
 	return 0;
 }
 

t/t7600-merge.sh

@@ -221,7 +221,7 @@ test_expect_success 'setup' '
 
 test_debug 'gitk --all'
 
-test_expect_failure 'test option parsing' '
+test_expect_success 'test option parsing' '
 	test_must_fail git merge -$ c1 &&
 	test_must_fail git merge --no-such c1 &&
 	test_must_fail git merge -s foobar c1 &&