Resolve GPDB_12_MERGE_FIXME in psql/command.c (#13207)
Resolve GPDB_12_MERGE_FIXME in psql/command.c (#13207)

In 6X_STABLE, \dP means "list parent-only tables", no children, but in PG_12_STABLE, \dP[tin+] means "list [only table/index] partitioned relations", they conflict.

This needs to be consistent with PG's behavior.
greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb