Convert git-clone script to python

[olevski]

At almost 200 lines and some not so simple logic all implemented in bash I think it may be a good idea to use python here.

This way we can write some unit tests for this. And it should make things a bit more easier to understand.

[Panaetius]

https://github.com/SwissDataScienceCenter/renku-notebooks/blob/master/git-clone/clone.sh

[Panaetius]

Evaluate if https://github.com/jelmer/dulwich is a good fit for this. Also evaluate if we can move BaseRepository et al. from renku-python to its own package and use it in both projects

[rokroskar]

done in #956