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>

reachable.c

@@ -341,7 +341,8 @@ static int mark_object_seen(const struct object_id *oid,
 			     int exclude UNUSED,
 			     uint32_t name_hash UNUSED,
 			     struct packed_git *found_pack UNUSED,
-			     off_t found_offset UNUSED)
+			     off_t found_offset UNUSED,
+			     void *payload UNUSED)
 {
 	struct object *obj = lookup_object_by_type(the_repository, oid, type);
 	if (!obj)