tests/test-dirstate-backup.t

diffordiffstat: avoid looking up contexts twice

I'm not worried about performance; this is just simpler.

Differential Revision: https://phab.mercurial-scm.org/D5890

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