HPR3548: Make a custom Git command

Summary: How to make your own Git subcommands with shell scripts, and then how to pass arguments to them with git rev-parse. For a real-world example of Git in scripts and hooks, see Git-portal. The joke site Klaatu mentions in the episode is git-man-page-generator. It's a joke site. Do not attempt to run these commands. However, it could be fun inspiration on what Git command you could make next!