Clean up dangling GitPython processes

[Panaetius]

Dangling GitPython subprocesses clogs up triples generation on vomcases during mgiration.

According to gitpython-developers/GitPython#1333 deleting the Repository object should clean up dangling processes, as it triggers garbage collection.

We should check if there's a way we can reduce the dangling git cat-file processes, either by deleting the Repository object at regular intervals and recreating it, or maybe killing child git cat-file processes in between processing commits (or every n commits).

Timeboxed to two days.

Inform KG team if this can be fixed.