pgsql: Fix MERGE command tag for actions blocked by BEFORE ROW triggers

Fix MERGE command tag for actions blocked by BEFORE ROW triggers.

This ensures that the row count in the command tag for a MERGE is
correctly computed in the case where UPDATEs or DELETEs are skipped
due to a BEFORE ROW trigger returning NULL (the INSERT case was
already handled correctly by ExecMergeNotMatched() calling
ExecInsert()).

Back-patch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCU8XEmR0JWKDtyb7iZ%3DqCffxS9uyJt0iOZ4TV4RT%2Bow1w%40mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/da6257eee35db5d281a115838abaf285b46b52f3

Modified Files
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src/backend/executor/nodeModifyTable.c | 10 ++++++----
src/test/regress/expected/merge.out | 15 +++++++++++++++
src/test/regress/sql/merge.sql | 13 +++++++++++++
3 files changed, 34 insertions(+), 4 deletions(-)