Handling unicode arguments

There is an issue with the handling of unicode arguments. This situation occurs when you have file named with unicode characters, that is returned correctly from GitPython as a Unicode type, if you then try and feed that back into the API it's not handled when unpacking the arguments.

I'm not sure how to make the repository encoding available at this point, so our fix currently just uses the default and we haven't run into anyone not using a repository with the default encoding.