(jameinel) Fix bug #830947. Dirstate.set_root_id() should update the
id_index correctly.
The symptom is that if you have a tree with the file_id TREE_ROOT somewhere
other than at '',
it could cause 'bzr co' to fail because it tries to lookup the file in the
id_index map,
but it incorrectly points to a location ('') where the id never existed.
(John A Meinel)