tests/test-dirstate-backup.t

githelp: don't reference 3rd party commands for `git show`

`hg show` is a Facebook-ism. Reference functionality in core.

The logic here isn't terrific. But it is better than nothing.

Differential Revision: https://phab.mercurial-scm.org/D1729

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