Move Branch.read_working_inventory to WorkingTree.
Branch.read_working_inventory has moved to WorkingTree.read_working_inventory. This necessitated changes to Branch.get_root_id, and a move of Branch.set_inventory to WorkingTree as well. To make it clear that a WorkingTree cannot always be obtained Branch.working_tree() will raise 'errors.NoWorkingTree' if one cannot be obtained. (Robert Collins)