pack-bitmap: allow passing payloads to `show_reachable_fn()`

pack-bitmap: allow passing payloads to `show_reachable_fn()`

The `show_reachable_fn` callback is used by a couple of functions to
present reachable objects to the caller. The function does not provide a
way for the caller to pass a payload though, which is functionality that
we'll require in a subsequent commit.

Change the callback type to accept a payload and adapt all callsites
accordingly.

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