tests/test-dirstate-backup.t

test: make sure sparse-revlog does not interfer with test-parseindex.t

That test carefully craft revlog to reproduce issues. We do not want new revlog
behavior to interfere with that.

Differential Revision: https://phab.mercurial-scm.org/D5314

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