svn merge -c 1541971 merging from trunk to branch-2 to fix HDFS-4995 Make getContentSummary less expensive.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1541975 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-MAPREDUCE-4884-branch-2-gd