COObjectGraphContext: tweak -setItemGraph: to only load items from the given graph if they are reachable from the root via a DFS. See -[TestBranch testCommittedGarbageObjectsNotReloaded] for the motivation.

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10290 7b3f36aa-e6db-49c1-8b4c-66eac1997e64