HDFS-4650. When passing two non-existing snapshot names to snapshotDiff, it returns success if the names are the same.  Contributed by Jing Zhao


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-2802@1476408 13f79535-47bb-0310-9956-ffa450edef68