parse-options: mark unused parameters in noop callback

parse-options: mark unused parameters in noop callback

Unsurprisingly, the noop options callback doesn't bother to look at any
of its parameters. Let's mark them so that -Wunused-parameter does not
complain.

Another option would be to drop the callback and have parse-options
itself recognize OPT_NOOP_NOARG. But that seems like extra work for no
real benefit.

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