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-branch.c b/builtin-branch.c
index 6aaa708..5687d60 100644
--- a/builtin-branch.c
+++ b/builtin-branch.c
@@ -610,7 +610,8 @@
 	}
 	hashcpy(merge_filter_ref, head_sha1);
 
-	argc = parse_options(argc, argv, options, builtin_branch_usage, 0);
+	argc = parse_options(argc, argv, prefix, options, builtin_branch_usage,
+			     0);
 	if (!!delete + !!rename + !!force_create > 1)
 		usage_with_options(builtin_branch_usage, options);