COBranch: use shorter form [self branchInfo] instead of [[_persistentRoot persistentRootInfo] branchInfoForUUID: _UUID]

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10258 7b3f36aa-e6db-49c1-8b4c-66eac1997e64