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.

Fixes #7

[ci skip]

More to come, especially when it's about strings

There is more work to do though, as many imports are still incorrect.
Also, there are still print statements

Commandline was
autopep8 -j 8 --max-line-length 120 --in-place --recursive --exclude "*gitdb*,*async*" git/

W291 trailing whitespace

E201 whitespace after '('
E202 whitespace before ')'
E203 whitespace before ':'
E225 missing whitespace around operator
E226 missing whitespace around arithmetic operator
E227 missing whitespace around bitwise or shift operator
E228 missing whitespace around modulo operator
E231 missing whitespace after ','
E241 multiple spaces after ','
E251 unexpected spaces around keyword / parameter equals

E301 expected 1 blank line, found 0
E302 expected 2 blank lines, found 1
E303 too many blank lines (n)