Fix bogus list-iteration code in pg_regress.c, affecting ecpg tests only.
Fix bogus list-iteration code in pg_regress.c, affecting ecpg tests only.

While looking at a recent buildfarm failure in the ecpg tests, I wondered
why the pg_regress output claimed the stderr part of the test failed, when
the regression diffs were clearly for the stdout part.  Looking into it,
the reason is that pg_regress.c's logic for iterating over three parallel
lists is wrong, and has been wrong since it was written: it advances the
"tag" pointer at a different place in the loop than the other two pointers.
Fix that.

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