tests(submodule): add new submodule commits
tests(submodule): add new submodule commits

It's somewhat more complex to add new commits in submodules to the
parent repository. The new test shows how to do that in a 'GitPythonic'
way.

Related to #335

jeblair/GitPython,h4ck3rm1k3/GitPython,breavo/GitPython,gitpython-developers/gitpython,expobrain/GitPython,cdht/GitPython,expobrain/GitPython,manasdk/GitPython,gitpython-developers/GitPython,nvie/GitPython,nvie/GitPython,manasdk/GitPython,beni55/GitPython,beni55/GitPython,gitpython-developers/GitPython,h4ck3rm1k3/GitPython,gitpython-developers/gitpython,cdht/GitPython,dpursehouse/GitPython,dpursehouse/GitPython,ghtdak/GitPython,cdht/GitPython,jeblair/GitPython,breavo/GitPython,ghtdak/GitPython