Code to map attribute numbers in map_partition_varattnos() duplicates
what convert_tuples_by_name_map() does.  Avoid that.

Amit Langote, per a report from Álvaro Herrera.

Discussion: http://postgr.es/m/9ce97382-54c8-deb3-9ee9-a2ec271d866b%40lab.ntt.co.jp