Properly adjust pointers when tuples are moved during CLUSTER.
Properly adjust pointers when tuples are moved during CLUSTER.

Otherwise, when we abandon incremental memory accounting and use
batch allocation for the final merge pass, we might crash.  This
has been broken since 0011c0091e886b874e485a46ff2c94222ffbf550.

Peter Geoghegan, tested by Noah Misch

adam8157/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,jmcatamney/gpdb,ashwinstar/gpdb