Implement `packed_delete_refs()` using a reference transaction. This means that `files_delete_refs()` can use `refs_delete_refs()` instead of `repack_without_refs()` to delete any packed references, decreasing the coupling between the classes. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>