OAK-928: Read access is enforced on NEW items

When a perfect copy of the shadowed content is added, the commit() becomes
a no-op and no write access checks get perfomed.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1545820 13f79535-47bb-0310-9956-ffa450edef68