tests/test-dirstate-backup.t

rust-hg-path: implement `Display` for `HgPath` and `HgPathBuf`

This is useful when debugging, to get a human readable output instead of an
array of `u8`.

Differential Revision: https://phab.mercurial-scm.org/D7523

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [255]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate