HIVE-5073: Fix problem with multiple root tasks in tez (Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/tez@1513379 13f79535-47bb-0310-9956-ffa450edef68
HIVE-5076: Subsequent reduce stages fail when executed in tez (Gunther Hagleitner)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/tez@1513612 13f79535-47bb-0310-9956-ffa450edef68