tests/test-dirstate-backup.t

test: use a more verbose output in the test

While debugging some test failure, I released the test never checks if the
relevant changesets were preserved. So I am updating the test from `hg parents`
usage to `hg log -G` with a special template. This increase the area covered by
the test and clarify the test failures.

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