OAK-768 : Enhance Root API in order to provide information about the ContentSession  (ooops... wrong merge of RootImpl#getPendingChanges. thanks michael for spotting)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1466977 13f79535-47bb-0310-9956-ffa450edef68