HADOOP-9078. enhance unit-test coverage of class org.apache.hadoop.fs.FileContext (Ivan A. Veselovsky via jeagles)

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svn merge -c 1532932 from trunk for HDFS-4376. Fix race conditions in Balancer.


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HDFS-5360. Improvement of usage message of renameSnapshot and deleteSnapshot. Contributed by Shinichi Yamashita.

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HDFS-5334. Merge change r1532949 from trunk.

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