Improve lib.git with branch

[zagy]

The current behaviour is a bit odd. git.Clone with branch makes sure the right version is there, but it resets the master.

@gforcada wrote:

One could do git clean -dfxq to get rid of all untracked files and then follow it with a regular git checkout BRANCH/REVISION.

[zagy]

@gforcada the git clean -x – I'm not sure about that. That is, I'm not against it, I just really don't know if that's a good idea. It would remove all untracked files, including the ignored ones. While is will allow a quite stable switch, it will, well, remove things. So it'll remove stuff like .pyc files. I can imagine, that this could delete too much.

One could also argue that this is a repo and there should not be anything in there.