RFC: Change clang-tidy rules

[cajun-rat]

At the moment we have a CI rule that requires all commits to be free of clang-tidy warnings, subject to the rules in .clang-tidy. This has three problems:

• Upgrades add new clang-tidy checks, and this rules makes upgrades harder. The outcome is to break the backwards compatibility guarantees that C++ provides.
• There is growing list of suppressed checks that would be nice to enforce on new code, but where existing code breaks them enough to make turning them on difficult. The outcome is that useful checks are currently disabled.
• Fixing the issues creates churn in otherwise stable code, which has the potential to introduce bugs.

I propose changing this to enforce a different rule: "The diff may not contain (new) clang-tidy matches". We should be able to enforce this using clang-tidy-diff.py, which is distributed in the Debian clang packages).