	Change the dependencies of a TopGit-controlled topic branch.
	This should have several subcommands, but only `add` is
	supported right now.

	The `add` subcommand takes an argument naming a topic branch to
	be added, adds it to '.topdeps', performs a commit and then
	updates your topic branch accordingly.  If you want to do other
	things related to the dependency addition, like adjusting
	'.topmsg', prepare them in the index before calling `tg depend
	add`.

	TODO: Subcommand for removing dependencies, obviously

