Investigate if it's possible to stop using git.GitCmdObjectDB

When instantiating git.Repo objects, we're currently selecting manually the odbt. This was done this way mainly because of these two:

https://github.com/gitpython-developers/GitPython/issues/51
https://github.com/gitpython-developers/GitPython/pull/58

Both issues should be fixed in the latest GitPython so we probably can just default to the "recommended" odbt.