MAPREDUCE-5065. DistCp should skip checksum comparisons if block-sizes are different on source/target. Contributed by Mithun Radhakrishnan.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1468629 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HDFS-4310-trunk-gd