tests/test-dirstate-backup.t

transplant: added support for --stop flag

This adds fuctionality for `--stop` flag to `transplant`.
A new method `stop` is added to `transplanter` class
containing logic to abort transplant.

Tests are updated as shown.

Differential Revision: https://phab.mercurial-scm.org/D6695

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