COBranch: don't create the branch's object graph context until it is used. This is a part of trying to speed up TestObjectGraphPerformance. Currently there is no speedup because the branch object graph contexts are created in various validation methods

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10411 7b3f36aa-e6db-49c1-8b4c-66eac1997e64