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.