new command: `vc unbranch`

no default Subversion implementation for a number of reasons:
-	it seems difficult to come up with a default implementation of %Mainline for Subversion
-	first of all, I probably should have made that an %info method rather than a directive
-	also, trunk/branches strategies in Subversion can vary wildly
-	but I think the first one is the more serious issue
-	I'll have to work on that, assuming I can manage to care about Subversion again :-/