diff: make -O and --output work in subdirectory

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -254,7 +254,8 @@ void diff_no_index(struct rev_info *revs,
 		else if (!strcmp(argv[i], "--"))
 			i++;
 		else {
-			j = diff_opt_parse(&revs->diffopt, argv + i, argc - i);
+			j = diff_opt_parse(&revs->diffopt, argv + i, argc - i,
+					   revs->prefix);
 			if (j <= 0)
 				die("invalid diff option/value: %s", argv[i]);
 			i += j;