Make 'rbt --help' be a little more sensical.

The help output for rbt would say "the most commonly-used commands are:" and
then proceed to spit out an unordered list of every command, no matter how
commonly used.

I've replaced the "most common" commands list with a curated list of the things
that I think are actually most common, in the order that people probably care
about them. We then print out the rest of the commands in alphabetical order.

Testing done: Ran rbt --help

Reviewed at https://reviews.reviewboard.org/r/4987/