parse-opts: prepare for OPT_FILENAME

To give OPT_FILENAME the prefix, we pass the prefix to parse_options()
which passes the prefix to parse_options_start() which sets the prefix
member of parse_opts_ctx accordingly. If there isn't a prefix in the
calling context, passing NULL will suffice.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin-mv.c b/builtin-mv.c
index 01270fe..8b81d4b 100644
--- a/builtin-mv.c
+++ b/builtin-mv.c
@@ -72,7 +72,8 @@
 	if (read_cache() < 0)
 		die("index file corrupt");
 
-	argc = parse_options(argc, argv, builtin_mv_options, builtin_mv_usage, 0);
+	argc = parse_options(argc, argv, prefix, builtin_mv_options,
+			     builtin_mv_usage, 0);
 	if (--argc < 1)
 		usage_with_options(builtin_mv_usage, builtin_mv_options);