OAK-659: Move purge logic for transient changes below the NodeBuilder interface

Remove the extra equals() check from AbstractRoot.rebase().
Fix the MutableNodeState.isModified() check to correctly handle the case where the modifications have been flushed.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1558270 13f79535-47bb-0310-9956-ffa450edef68