make a little tool to catch API changes between commits

During releases we need a good way of creating release notes for API changes, both that we directly introduce, as well as Core's changes with their underlying refactors upstream.

One idea could be to have a little tool that calls:
`bitcoin-cli help` of both commits, displays a diff,
then takes that output, takes the commands listed, calls help on those, then compares the diff for both.

Script would take two hashes/tags as arguments, git clone, compile, etc, everything, start daemons, etc.