MAPREDUCE-5009. Killing the Task Attempt slated for commit does not clear the value from the Task commitAttempt member (Robert Parker via jeagles)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1447969 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd