refs.c: use is_dir_sep() in resolve_gitlink_ref()

The "submodule" argument in this function is a path, which can have
either '/' or '\\' as a separator. Use is_dir_sep() to support both.

Noticed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1507,7 +1507,7 @@ int resolve_gitlink_ref(const char *submodule, const char *refname,
 	struct ref_store *refs;
 	int flags;
 
-	while (len && submodule[len - 1] == '/')
+	while (len && is_dir_sep(submodule[len - 1]))
 		len--;
 
 	if (!len)