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>

hash-object.c

@@ -84,7 +84,8 @@ int main(int argc, const char **argv)
 
 	git_extract_argv0_path(argv[0]);
 
-	argc = parse_options(argc, argv, hash_object_options, hash_object_usage, 0);
+	argc = parse_options(argc, argv, NULL, hash_object_options,
+			     hash_object_usage, 0);
 
 	if (write_object) {
 		prefix = setup_git_directory();