OAK-853: Many child nodes: Diffing causes many calls to MicroKernel.getNodes

Optimization for the x.compareAgainstBaseState(x, ...) special case

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1489341 13f79535-47bb-0310-9956-ffa450edef68