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-archive.c

@@ -80,7 +80,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
-	argc = parse_options(argc, argv, local_opts, NULL, PARSE_OPT_KEEP_ALL);
+	argc = parse_options(argc, argv, prefix, local_opts, NULL,
+			     PARSE_OPT_KEEP_ALL);
 
 	if (output)
 		create_output_file(output);