	Make a commit on the current TopGit-controlled topic branch
	that makes it equal to its base, including the presence or
	absence of .topmsg and .topdeps.  Annihilated branches are not
	displayed by `tg summary`, so they effectively get out of your
	way.  However, the branch still exists, and `tg push` will
	push it (except if given the '-a' option).  This way, you can
	communicate that the branch is no longer wanted.

	Normally, this command will remove only empty branch
	(base == head, except for changes to the .top* files); use
	'-f' to annihilate a non-empty branch.

