HADOOP-8545. Fixed merge to branch-2.

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Merge -c 1527048 from trunk to branch-2 to fix MAPREDUCE-5529. Fix compat with hadoop-1 in mapred.TotalOrderPartitioner by re-introducing (get,set)PartitionFile which takes in JobConf. Contributed by Robert Kanter.

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YARN-1021. Yarn Scheduler Load Simulator. (ywskycn via tucu)

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Fixing up CHANGES.txt for HDFS-5031.

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HDFS-5268. Merging change r1527087 from trunk

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HDFS-5265. Merge change r1527093 from trunk.

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HDFS-5260. Merging change r1527113 from trunk to branch-2.

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