Every CLI program should have a usage message.
Also add a mention in the `push-to-pull-request` usage message of
its participation in the `refs/remotes/pr/` pseudo-remote feature.
This directory contains scripts that are used in building, managing,
testing, and other forms of work in a Zulip development environment.
Note that tools that are also useful in production belong in
scripts/ or should be Django management commands.