Fix cross-leftover pollution in FOR PORTION OF insert triggers

Fix cross-leftover pollution in FOR PORTION OF insert triggers

When we insert temporal leftovers after an UPDATE FOR PORTION OF, we
must make a new copy of the tuple before each insert.  Otherwise, if
an insert trigger assigns to attributes of NEW, the second leftover
sees those changes.

Author: Sergei Patiakin <sergei.patiakin@enterprisedb.com>
Reviewed-by: Paul A Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/flat/CANE55rCqcse_pwXBMWhbj3_7XROb8Dks6%3DOLFmKy3bO3zDsCsg%40mail.gmail.com