HDFS-4648. For snapshot deletion, when merging the diff from to-delete snapshot to the prior snapshot, make sure files/directories created after the prior snapshot get deleted.  Contributed by Jing Zhao


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1462865 13f79535-47bb-0310-9956-ffa450edef68