pgsql: Fix AFTER ROW trigger execution in MERGE cross-partition update.

Fix AFTER ROW trigger execution in MERGE cross-partition update.

When executing a MERGE UPDATE action, if the UPDATE is turned into a
cross-partition DELETE then INSERT, do not attempt to invoke AFTER
UPDATE ROW triggers, or any of the other post-update actions in
ExecUpdateEpilogue().

For consistency with a plain UPDATE command, such triggers should not
be fired (and typically fail anyway), and similarly, other post-update
actions, such as WCO/RLS checks should not be executed, and might also
lead to unexpected failures.

Therefore, as with ExecUpdate(), make ExecMergeMatched() return
immediately if ExecUpdateAct() reports that a cross-partition update
was done, to be sure that no further processing is done for that
tuple.

Back-patch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCWjBgagyNZs02vgDF0DvASYj-iHTFtXG2-nP3orZhmtcw%40mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/06a546382aafe0cc9d895b2861c5b8a8e9f0b002

Modified Files
--------------
src/backend/executor/nodeModifyTable.c | 16 ++++++++++++
src/test/regress/expected/triggers.out | 45 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/triggers.sql      | 36 +++++++++++++++++++++++++++
3 files changed, 97 insertions(+)