Code review comment for lp:~mbp/bzr/368931-rename-case-2.0

This finishes off Chris Jones's patch in https://bugs.edge.launchpad.net/bzr/+bug/368931 by adding a test.

This function could be faster but I'm not sure if it's actually a hot spot.

This finishes our last high-priority bug with a patch! There are still 34 more with lower importance.