ref-filter: 'contents:trailers' show error if `:` is missing

The 'contents' atom does not show any error if used with 'trailers'
atom and colon is missing before trailers arguments.

e.g %(contents:trailersonly) works, while it shouldn't.

It is definitely not an expected behavior.

Let's fix this bug.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Heba Waly <heba.waly@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ref-filter.c

@@ -342,9 +342,11 @@ static int contents_atom_parser(const struct ref_format *format, struct used_ato
 		atom->u.contents.option = C_SIG;
 	else if (!strcmp(arg, "subject"))
 		atom->u.contents.option = C_SUB;
-	else if (skip_prefix(arg, "trailers", &arg)) {
-		skip_prefix(arg, ":", &arg);
-		if (trailers_atom_parser(format, atom, *arg ? arg : NULL, err))
+	else if (!strcmp(arg, "trailers")) {
+		if (trailers_atom_parser(format, atom, NULL, err))
+			return -1;
+	} else if (skip_prefix(arg, "trailers:", &arg)) {
+		if (trailers_atom_parser(format, atom, arg, err))
 			return -1;
 	} else if (skip_prefix(arg, "lines=", &arg)) {
 		atom->u.contents.option = C_LINES;

t/t6300-for-each-ref.sh

@@ -785,6 +785,14 @@ test_expect_success '%(trailers) rejects unknown trailers arguments' '
 	test_i18ncmp expect actual
 '
 
+test_expect_success 'if arguments, %(contents:trailers) shows error if colon is missing' '
+	cat >expect <<-EOF &&
+	fatal: unrecognized %(contents) argument: trailersonly
+	EOF
+	test_must_fail git for-each-ref --format="%(contents:trailersonly)" 2>actual &&
+	test_i18ncmp expect actual
+'
+
 test_expect_success 'basic atom: head contents:trailers' '
 	git for-each-ref --format="%(contents:trailers)" refs/heads/master >actual &&
 	sanitize_pgp <actual >actual.clean &&