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-config.c b/builtin-config.c
index a81bc8b..60915f9 100644
--- a/builtin-config.c
+++ b/builtin-config.c
@@ -316,7 +316,8 @@
 
 	config_exclusive_filename = getenv(CONFIG_ENVIRONMENT);
 
-	argc = parse_options(argc, argv, builtin_config_options, builtin_config_usage,
+	argc = parse_options(argc, argv, prefix, builtin_config_options,
+			     builtin_config_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 
 	if (use_global_config + use_system_config + !!given_config_file > 1) {