Tooling for verification of PGP signed commits

This is an incomplete work in progress, but currently includes a pre-push hook script (pre-push-hook.sh) for maintainers to ensure that their own commits are PGP signed (nearly always merge commits), as well as a script to verify commits against a trusted keys list.

Using verify-commits.sh safely

Remember that you can't use an untrusted script to verify itself. This means that checking out code, then running verify-commits.sh against HEAD is not safe, because the version of verify-commits.sh that you just ran could be backdoored. Instead, you need to use a trusted version of verify-commits prior to checkout to make sure you're checking out only code signed by trusted keys:

git fetch origin && \
  ./contrib/verify-commits/verify-commits.sh origin/master && \
  git checkout origin/master

Note that the above isn't a good UI/UX yet, and needs significant improvements to make it more convenient and reduce the chance of errors; pull-reqs improving this process would be much appreciated.