tests/test-dirstate-backup.t

unfinishedstate: suggested `hg update .` (including `.`) to complete update

`hg update` can update to a different and undesired commit. For users
who have commands.update.requiredest=yes, it's even an error to run
just `hg update.

Differential Revision: https://phab.mercurial-scm.org/D6956

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