GitPython in submodules

Using GitPython in a submodule doesn't work.

Python 2.7.6, GitPython 0.3.2 RC1 on Windows.

I have a repository 'a' with a submodule 'b'. I do:

```
>>> repo = git.Repo('C:\a\b')
>>> repo
(git.Repo "C:\a\.git")
>>> # (should be "C:\a\.git\modules\b")
>>> repo.heads
(... a's branches...)
```
