pgsql: Fix MERGE command tag for cross-partition updates.

Fix MERGE command tag for cross-partition updates.

This ensures that the row count in the command tag for a MERGE is
correctly computed. Previously, if MERGE updated a partitioned table,
the row count would be incorrect if any row was moved to a different
partition, since such updates were counted twice.

Back-patch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCWRMG7XX2QEsVL1LswmNo2d_YG8tKTLkpD3=Lp644S7rg@mail.gmail.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/018af1cc1c8075346e6a5fe3bb77b7e31399be70

Modified Files
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src/backend/executor/nodeModifyTable.c |  2 +-
src/test/regress/expected/merge.out    | 16 ++++++++++++++++
src/test/regress/sql/merge.sql         | 11 +++++++++++
3 files changed, 28 insertions(+), 1 deletion(-)