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-describe.c b/builtin-describe.c
index 63c6a19..7a66298 100644
--- a/builtin-describe.c
+++ b/builtin-describe.c
@@ -322,7 +322,7 @@
 		OPT_END(),
 	};
 
-	argc = parse_options(argc, argv, options, describe_usage, 0);
+	argc = parse_options(argc, argv, prefix, options, describe_usage, 0);
 	if (max_candidates < 0)
 		max_candidates = 0;
 	else if (max_candidates > MAX_TAGS)