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

[nbarrientos]

When instantiating git.Repo objects, we're currently selecting manually the odbt. This was done this way mainly because of these two:

gitpython-developers/GitPython#51
gitpython-developers/GitPython#58

Both issues should be fixed in the latest GitPython so we probably can just default to the "recommended" odbt.