externalise dependency on git

[Panaetius]

we currently depend on gitpython directly in many places, but ideally git would just be an implementation detail.

We should have an interface that defines certain common operations that we use git for that then has a git specific implementation that gets injected as a dependency, so our code only depends on the interface.

As such, the interface should have methods for "git add", "git commit", "git push" directly, but should potentially combine those to units that make sense from our code (e.g. add and commit in a single method called persist_file_changes), in a way that it could still be used with a different implementation of the interface, like if we wanted to use subversion as a backend (not that we'd ever do this...).

[Panaetius]

We should also consider moving away from GitPython. iterative/dvc#2215 has some very similar considerations and might be a good starting point for this.