pgsql: Allow using the updated tuple while moving it to a differentpar

Allow using the updated tuple while moving it to a different partition.

An update that causes the tuple to be moved to a different partition was
missing out on re-constructing the to-be-updated tuple, based on the latest
tuple in the update chain.  Instead, it's simply deleting the latest tuple
and inserting a new tuple in the new partition based on the old tuple.
Commit 2f17844104 didn't consider this case, so some of the updates were
getting lost.

In passing, change the argument order for output parameter in ExecDelete
and add some commentary about it.

Reported-by: Pavan Deolasee
Author: Amit Khandekar, with minor changes by me
Reviewed-by: Dilip Kumar, Amit Kapila and Alvaro Herrera
Backpatch-through: 11
Discussion: https://postgr.es/m/CAJ3gD9fRbEzDqdeDq1jxqZUb47kJn+tQ7=Bcgjc8quqKsDViKQ@mail.gmail.com

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0527df732beb8c2d584c7e4ea5aeb05cf92e14a2

Modified Files
--------------
src/backend/commands/trigger.c                     | 22 ++++++-
src/backend/executor/execReplication.c             |  2 +-
src/backend/executor/nodeModifyTable.c             | 73 +++++++++++++++------
src/include/commands/trigger.h                     |  3 +-
.../isolation/expected/partition-key-update-4.out  | 60 +++++++++++++++++
src/test/isolation/isolation_schedule              |  1 +
.../isolation/specs/partition-key-update-4.spec    | 76 ++++++++++++++++++++++
7 files changed, 214 insertions(+), 23 deletions(-)