Deprecate GitPython

GitPython is not compatible with python3 and may never be:
https://fedoraproject.org/wiki/User:Churchyard/python3

GitPython was only used in the test modules, and this
branch switches to straight-up git commands to
clear the way for additional (future) python3 compat changes.

After these commits:

* `./runtests.py` is still clean on:
   - python 2.4 (centos 5.9)
   - python 2.6 (centos 6.4)
   - python 2.7 (fedora 20)

* `python3 ./runtests.py test/unit` is still clean on:
   - python 3.3 (fedora 20)

____

Note: We're still not python3-compatible.
Further compat changes are needed for python3, but
i'm running into encoding issues that i need to resolve 
in a different PR.