Add tests for pre-commit hooks

[klamann]

Make sure that

• the pre-commit hooks work as expected
• that all files of a new repo pass the pre-commit hook conditions (no errors on the first commit)

This turned out to be harder than expected, because pre-commit will only run when we have a git repo and all files we want to check have been added to the git repo, which is pretty slow, considering that we want to do this in a loop on a lot of repos.

related PR: #6