Race condition when spawning git processes for autograding

[meffmadd]

Describe the bug
The bug is caused by the tornado Subprocess.wait_for_exit class: tornadoweb/tornado#3364
https://github.com/TU-Wien-dataLAB/Grader-Service/blob/baaaca0da029c881a01423a34f5b1d5f6620dae0/grader_service/autograding/local_grader.py#L440

I noticed this when testing autograding in the Celery workers where it would hang on git init.

This could also affect the git server:
https://github.com/TU-Wien-dataLAB/Grader-Service/blob/baaaca0da029c881a01423a34f5b1d5f6620dae0/grader_service/handlers/git/server.py#L48

I switched to Popen for the Celery branch, which is a drop-in replacement.