Hey thanks, Jimmy and Jacob.
I guess I just have to learn all about how git to manage multiple
copies of my code in sync. I think that means I should have a big git
repository that contains all my little git repositories, and learn
how to use git properly.
As to the Unix ln command, I'd use it if it would work on
directories. According to this page,
Darwin breaks Posix standards and allows hard links to directories in
the HFS+ filesystem. But I can't get it to work. Maybe they've since
killed that idea because it's potentially dangerous.