pack-bitmap: convert traverse_bitmap_commit_list to object_id

Convert traverse_bitmap_commit_list and the callbacks it takes to use a
pointer to struct object_id.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.h

@@ -27,7 +27,7 @@ enum pack_bitmap_flags {
 };
 
 typedef int (*show_reachable_fn)(
-	const unsigned char *sha1,
+	const struct object_id *oid,
 	enum object_type type,
 	int flags,
 	uint32_t hash,