The es_root_result_relations array needs to be shallow-copied in the
same way as the main es_result_relations array, else EPQ rechecks on
partitioned result relations fail, as seen in bug #15677 from
Norbert Benkocs.

Amit Langote, isolation test case added by me

Discussion: https://postgr.es/m/15677-0bf089579b4cd02d@postgresql.org
Discussion: https://postgr.es/m/19321.1554567786@sss.pgh.pa.us