Merge branch 'mh/ref-locking-fix'

Transactions to update multiple references that involves a deletion
was quite broken in an error codepath and did not abort everything
correctly.

* mh/ref-locking-fix:
  files_transaction_prepare(): fix handling of ref lock failure
  t1404: add a bunch of tests of D/F conflicts

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