	Change 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 of 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

