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>

builtin-checkout-index.c

@@ -249,7 +249,7 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 		die("invalid cache");
 	}
 
-	argc = parse_options(argc, argv, builtin_checkout_index_options,
+	argc = parse_options(argc, argv, prefix, builtin_checkout_index_options,
 			builtin_checkout_index_usage, 0);
 	state.force = force;
 	state.quiet = quiet;