Easier clang format script and git-hook-able version
After my last failure on this topic (!509 (closed), sorry!), now comes the real PR.
This PR adds to things:
- it makes the clang-format-local script a bit more readable, adds support for non-root users (otherwise the changes files are always owned by the root-user afterwards) and uses a defined working directory in the docker container (instead of the one the user gave)
- There is now also a version of this script, which only checks the files added to the git staging area and re-adds them after fixing with clang-format. This may help users to submit things without having to wait for the CI to dismiss their style :-)