Merge branch 'mh/packed-ref-transactions'

Merge branch 'mh/packed-ref-transactions'

Implement transactional update to the packed-ref representation of
references.

* mh/packed-ref-transactions:
  files_transaction_finish(): delete reflogs before references
  packed-backend: rip out some now-unused code
  files_ref_store: use a transaction to update packed refs
  t1404: demonstrate two problems with reference transactions
  files_initial_transaction_commit(): use a transaction for packed refs
  prune_refs(): also free the linked list
  files_pack_refs(): use a reference transaction to write packed refs
  packed_delete_refs(): implement method
  packed_ref_store: implement reference transactions
  struct ref_transaction: add a place for backends to store data
  packed-backend: don't adjust the reference count on lock/unlock