Incorrect command tag row count for MERGE with a cross-partition update

Playing around with MERGE some more, I noticed that the command tag
row count is wrong if it does a cross-partition update:

CREATE TABLE target (a int, b int) PARTITION BY LIST (b);
CREATE TABLE target_p1 PARTITION OF target FOR VALUES IN (1);
CREATE TABLE target_p2 PARTITION OF target FOR VALUES IN (2);
INSERT INTO target VALUES (1,1);

MERGE INTO target t USING (VALUES (1)) v(a) ON t.a = v.a
WHEN MATCHED THEN UPDATE SET b = 2;

which returns "MERGE 2" when only 1 row was updated, because
ExecUpdateAct() will update estate->es_processed for a cross-partition
update (but not for a normal update), and then ExecMergeMatched() will
update it again.

I think the best fix is to have ExecMergeMatched() pass canSetTag =
false to ExecUpdateAct(), so that ExecMergeMatched() takes
responsibility for updating estate->es_processed in all cases.

Regards,
Dean