Prevent invalid rules from being committed, or at leat warn when they are

Sometimes people commit rules that we can automatically detect are invalid, for example with utils/trivial-validate.py. It would be good to have a system to prevent these from being committed at all or to immediately warn someone when they have been, rather than having to wait until someone tries to build the extension later.

I've never used commit-hooks, so I don't know exactly what options are available in the git infrastructure to help with this. Another alternative is a daemon that periodically runs validation scripts across the current tree and generates e-mail if anything invalid is present.