Tests/Core/TestPersistentRoot.m: Add a trivial test of COPersistentRoot.currentRevision for uncommitted persistent root is nill

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10208 7b3f36aa-e6db-49c1-8b4c-66eac1997e64