pack-objects: convert packlist_find() to use object_id

We take a raw hash pointer, but most of our callers have a "struct
object_id" already. Let's switch to taking the full struct, which will
let us continue removing uses of raw sha1 buffers.

There are two callers that do need special attention:

  - in rebuild_existing_bitmaps(), we need to switch to
    nth_packed_object_oid(). This incurs an extra hash copy over
    pointing straight to the mmap'd sha1, but it shouldn't be measurable
    compared to the rest of the operation.

  - in can_reuse_delta() we already spent the effort to copy the sha1
    into a "struct object_id", but now we just have to do so a little
    earlier in the function (we can't easily convert that function's
    callers because they may be pointing at mmap'd REF_DELTA blocks).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-objects.h

@@ -187,7 +187,7 @@ struct object_entry *packlist_alloc(struct packing_data *pdata,
 				    uint32_t index_pos);
 
 struct object_entry *packlist_find(struct packing_data *pdata,
-				   const unsigned char *sha1,
+				   const struct object_id *oid,
 				   uint32_t *index_pos);
 
 static inline uint32_t pack_name_hash(const char *name)