Provide a command to switch a repository between trees and no working trees

When creating a repository you have the choice of having trees in the branches inside it, and having no trees.

There is currently no clean way to switch a created repository between the two settings. At the moment it is controlled by a single file in the .bzr dir, but users shouldn't have to edit things in there directly.

This spec proposes that there is a command that will make the switch for the user.