Avoid mutliple truncates for AOCO during vacuum.
Avoid mutliple truncates for AOCO during vacuum.

Vacuum for AOCO happens in mutliple phases. Prepare phase always truncates the
file to logical EOF. So, there is no need for redundant calls in later phases.

Signed-off-by: Ashwin Agrawal <924bf3437ad315c1c74f9c41d83153600b05f548@pivotal.io>

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