OAK-565 - CommitCommand readExistingNodes/mergeNodes called unnecessarily

Avoid unnecessary read and merge of head revision nodes. This fix needs to be revisited as OAK-571 is fixed.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1436344 13f79535-47bb-0310-9956-ffa450edef68