tests/test-dirstate-backup.t

remotenames: work around move of ABCs in collections

This starts warning in Python 3.7, and will break in 3.8. The import
is performed in a curious place because `import collections.abc`
explodes badly on Python 2.7, and `from collections import abc` flunks
our import checker.

Differential Revision: https://phab.mercurial-scm.org/D2847

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