Add optional directory argument to `cd` command

[overhacked]

Adds the ability to specify an optional second argument to the homeshick cd command that will change directory into the castle repo's subdirectory that corresponds to the given directory, as long as it is within $HOME and is tracked by the castle.

Most convenient to use when $PWD is within $HOME:

homeshick cd CASTLE .

Other changes

• Some fixes that ensure test suite can run on very recent (late 2022) versions of Git and Bash.
• Improvements to error reporting of command line usage errors.

[andsens]

OK, so this is in part on me for not documenting the feature better, but you can already do this:

The $PWD feature is neat, but without some strong usecases I don't see the justification for the additional code. It's just not something I imagine one would use very often.

All of that being said. Beautiful PR! With tests and everything :-)

Perhaps you could make a new PR to extend the helptext to include the homeshick cd CASTLE[/DIR] usage?