Merge branch 'kn/refs-files-case-insensitive'

Merge branch 'kn/refs-files-case-insensitive'

Deal more gracefully with directory / file conflicts when the files
backend is used for ref storage, by failing only the ones that are
involved in the conflict while allowing others.

* kn/refs-files-case-insensitive:
  refs/files: handle D/F conflicts during locking
  refs/files: handle F/D conflicts in case-insensitive FS
  refs/files: use correct error type when lock exists
  refs/files: catch conflicts on case-insensitive file-systems