OAK-1063: MutableTree.enter() simplification

Make exists() and a few other public methods call enter() so that the move tracking code is just in one place.
Inline the super.getPath() call in getPathInternal() so we can replace the getParent() call with direct parent access.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1528472 13f79535-47bb-0310-9956-ffa450edef68