A maintainer comments /bot fix-tidy on a PR. A read-only job runs
scripts/run_clang_tidy.sh --fix on the PR's merge commit and returns the
patch; a second job, which never runs the PR's code, accepts it only when
it adds nothing but #include lines to C++ files the PR already changes,
then commits it on the PR head and pushes with CLANG_TIDY_BOT_TOKEN.