Merge branch 'ps/refs-files-remove-empty-parent'

When a ref creation at refs/heads/foo/bar fails, the files backend
now removes refs/heads/foo/ if the directory is otherwise not used.

* ps/refs-files-remove-empty-parent:
  refs/files: remove empty parent dirs when ref creation fails

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

diff --cc t/t1400-update-ref.sh
Simple merge