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

The 'contents' atom does not show any error if used with 'trailers'
atom and semicolon 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>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>

ref-filter.c

@@ -332,6 +332,22 @@ static int trailers_atom_parser(const struct ref_format *format, struct used_ato
 	return 0;
 }
 
+static int check_format_field(const char *arg, const char *field, const char **option)
+{
+	const char *opt;
+	if (skip_prefix(arg, field, &opt)) {
+		if (*opt == '\0') {
+			*option = NULL;
+			return 1;
+		}
+		else if (*opt == ':') {
+			*option = ++opt;
+			return 1;
+		}
+	}
+	return 0;
+}
+
 static int contents_atom_parser(const struct ref_format *format, struct used_atom *atom,
 				const char *arg, struct strbuf *err)
 {
@@ -345,9 +361,8 @@ 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 (check_format_field(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

@@ -823,6 +823,15 @@ test_expect_success '%(trailers) rejects unknown trailers arguments' '
 	test_i18ncmp expect actual
 '
 
+test_expect_success 'if arguments, %(contents:trailers) shows error if semicolon is missing' '
+	# error message cannot be checked under i18n
+	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 &&