HDFS-4407. Change INodeDirectoryWithSnapshot.Diff.combinePostDiff(..) to merge-sort like and keep the postDiff parameter unmodified.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1433918 13f79535-47bb-0310-9956-ffa450edef68