Add flag to pg_regress to display failure diffs on stdout.
Add flag to pg_regress to display failure diffs on stdout.

When given --print-failure-diffs, pg_regress prints the contents of
regression.diffs after a failure.

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