reftable/basics: introduce `REFTABLE_UNUSED` annotation

reftable/basics: introduce `REFTABLE_UNUSED` annotation

Introduce the `REFTABLE_UNUSED` annotation and replace all existing
users of `UNUSED` in the reftable library to use the new macro instead.

Note that we unconditionally define `MAYBE_UNUSED` in the exact same
way, so doing so unconditionally for `REFTABLE_UNUSED` should be fine,
too.

Suggested-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>