If a git repo has the misfortune to have a file with the name "HEAD"
at the root level of the repo, git will return an error because it
is unsure whether the file or ref is meant:

    File "/usr/local/lib/python2.7/dist-packages/git/refs/head.py", line 81, in reset
      self.repo.git.reset(mode, commit, add_arg, paths, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/git/cmd.py", line 440, in <lambda>
      return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
    File "/usr/local/lib/python2.7/dist-packages/git/cmd.py", line 834, in _call_process
      return self.execute(make_call(), **_kwargs)
    File "/usr/local/lib/python2.7/dist-packages/git/cmd.py", line 627, in execute
      raise GitCommandError(command, status, stderr_value)
  GitCommandError: 'git reset --hard HEAD' returned with exit code 128
  stderr: 'fatal: ambiguous argument 'HEAD': both revision and filename
  Use '--' to separate filenames from revisions'

Implement its suggested fix by always passing '--' as an argument to
"git reset".  It is fine to pass it with no file specifiers afterwords.
In that case, git knows that "HEAD" is always meant as the ref.