We should write step-by-step instructions so we can point people to it, instead of having each new contributor figure out how to invoke `clang-format-diff.py` properly.
This how-to guide should cover:
- If clang-format needs to be on your PATH or not.
- How to apply the formatting non-destructively, check the changes, and then collapse the changes properly. Sometimes clang-format is too aggressive and touches lines which are not changed by a patch.
- How to format all the commits in a PR.