Add git precommit with clang-format

[jiridanek]

This follows up to

• Initial clang-format work: config file and git-clang-format instructions #504

[jiridanek]

There are two possibilities, either write the hook from scratch, or use one of the hook frameworks

• https://github.com/sds/overcommit
• https://github.com/pre-commit/pre-commit
• see https://githooks.com/ for longer list

[jiridanek]

https://github.com/pre-commit/pre-commit only works on the file-level, that is, check succeeds only if the whole file is properly formatted. What we want (for now) is to only check formatting of the changed lines.