OAK-2160: mk.getRevisionHistory: clarify since parameter

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1629148 13f79535-47bb-0310-9956-ffa450edef68
OAK-2161: MicroKernelIT: clean test content

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1629160 13f79535-47bb-0310-9956-ffa450edef68