Fix handling of oldtuple to match PG17 upstream (#7340)

A [previous
commit](d7513b2)
I made added support for MERGE ... RETURNING ....
While working on the coverity fix, I realized that this code handling
oldtuple was not added; that's what this fixes.

Disable-check: force-changelog-file

src/import/ht_hypertable_modify.c

@@ -461,6 +461,12 @@ lmerge_matched:;
 	 * visible to our MVCC snapshot.
 	 */
 
+#if PG17_GE
+	if (oldtuple != NULL)
+		ExecForceStoreHeapTuple(oldtuple, resultRelInfo->ri_oldTupleSlot,
+								false);
+	else
+#endif
 	if (!table_tuple_fetch_row_version(resultRelInfo->ri_RelationDesc,
 					   tupleid,
 					   SnapshotAny,