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>

pack-bitmap.h

@@ -50,7 +50,8 @@ typedef int (*show_reachable_fn)(
 	int flags,
 	uint32_t hash,
 	struct packed_git *found_pack,
-	off_t found_offset);
+	off_t found_offset,
+	void *payload);
 
 struct bitmap_index;