builtins: always pass prefix to parse_options()

Our builtins receive a "prefix" argument as part of their cmd_foo()
function. We should always pass this to parse_options() if we're calling
it, as it may be used for OPT_FILENAME() options.

In the cases here, there's no option that would use it, so we're not
fixing any bug. This is just future-proofing and setting a good example
(plus quelling some -Wunused-parameter warnings).

Note in the case of revert/cherry-pick, that we plumb the prefix through
to run_sequencer(), as those builtins are just thin wrappers around it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mktag.c

@@ -81,7 +81,7 @@ int cmd_mktag(int argc, const char **argv, const char *prefix)
 	int tagged_type;
 	struct object_id result;
 
-	argc = parse_options(argc, argv, NULL,
+	argc = parse_options(argc, argv, prefix,
 			     builtin_mktag_options,
 			     builtin_mktag_usage, 0);