pre-commit picks up local files not tracked by git

[MakisH]

See comment by @davidscn in #705 (comment)

pre-commit just executes this script, which does not distinguish if the files are staged or not: https://github.com/precice/tutorials/blob/develop/tools/check-image-names.py

The rest of the custom scripts we use should show the same behavior.

We could either guard the scripts, or maybe there are options in pre-commit.

[davidscn]

We could either guard the scripts, or maybe there are options in pre-commit.

I think we could just ask git to give us a list of files.