tests/test-dirstate-backup.t

rust: run `cargo clippy`

These automatic fixes are good to have because they make the code
more idiomatic and less surprising.

The transform from `sort` -> `sort_unstable` is questionable, but this is
only in a test, so it doesn't matter in our case.

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
  [10]

No dirstate backups are left behind

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