Merge branch 'ab/refs-files-cleanup'

Continued work on top of the hn/refs-errno-cleanup topic.

* ab/refs-files-cleanup:
  refs/files: remove unused "errno != ENOTDIR" condition
  refs/files: remove unused "errno == EISDIR" code
  refs/files: remove unused "oid" in lock_ref_oid_basic()
  refs API: remove OID argument to reflog_expire()
  reflog expire: don't lock reflogs using previously seen OID
  refs/files: add a comment about refs_reflog_exists() call
  refs: make repo_dwim_log() accept a NULL oid
  refs/debug: re-indent argument list for "prepare"
  refs/files: remove unused "skip" in lock_raw_ref() too
  refs/files: remove unused "extras/skip" in lock_ref_oid_basic()
  refs: drop unused "flags" parameter to lock_ref_oid_basic()
  refs/files: remove unused REF_DELETING in lock_ref_oid_basic()
  refs/packet: add missing BUG() invocations to reflog callbacks

diff --cc refs/debug.c
Simple merge

diff --cc refs/files-backend.c
Simple merge