Make format check pipeline only react on diff'd files

[mortbopet]

Currently, the format check pipeline will check the entire repo for out-of-format files. This is an issue for PRs which may have failing format PR gates without having touched other files in the repo, if those files somehow have drifted away from the accepted format.

For clang-format, i would recommend to borrow/steal the steps here which runs clang-format on the git diff, and prints/uploads patches.