Python 3 didn't like some of the stuff that we were doing with
``subprocess.check_call()`` in these tests, so to fix this I have redone
that stuff with calls to functions in the git execution module. In order
to avoid problems with running tests with no global gitconfig, I needed
to add an argument called ``git_opts`` to most of the funcs in the git
execution module (well I didn't *need* to do it to most of the funcs, it
just seemed like we shouldn't only be supporting this argument in a
single function).
This new ``git_opts`` argument is specifically for passing arguments to
the git command itself (not the subcommand). For example, ``git -c
user.name="Foo Bar" commit .....`` is different than running ``git
commit -c user.name="Foo Bar" .....``, because the ``commit`` subcommand
for git also accepts ``-c``.
