tests/test-dirstate-backup.t

hbisect: use a revset for ancestor calculation

Since we have revsets we can be more concise in doing the ancestor calulcation.
Significant commits are all descendent of the topmost good commits.

Test Plan:
    python run-tests.py test-bisect*

Differential Revision: https://phab.mercurial-scm.org/D1498

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