Merge HDFS-5331 make SnapshotDiff.java to a o.a.h.util.Tool interface implementation. Contributed by Vinay.



git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1533918 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd