tests/test-dirstate-backup.t

test: drop usage of `HGMERGE` in `test-lfconvert.t`

Using configuration instead of environment variable has a strange effect on
the output, but no impact on the end result.

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