Fix one-off bug causing missing commit timestamps for subtransactions

The logic in charge of writing commit timestamps (enabled with
track_commit_timestamp) for subtransactions had a one-bug bug,
where it would be possible that commit timestamps go missing for the
last subtransaction committed.

While on it, simplify a bit the iteration logic in the loop writing the
commit timestamps, as per suggestions from Kyotaro Horiguchi and Tom
Lane, so as some variable initializations are not part of the loop
itself.

Issue introduced in 73c986a.

Analyzed-by: Alex Kingsborough
Author: Alex Kingsborough, Kyotaro Horiguchi
Discussion: https://postgr.es/m/73A66172-4050-4F2A-B7F1-13508EDA2144@amazon.com
Backpatch-through: 10

src/backend/access/transam/commit_ts.c

@@ -168,7 +168,9 @@ TransactionTreeSetCommitTsData(TransactionId xid, int nsubxids,
 	 * subxid not on the previous page as head.  This way, we only have to
 	 * lock/modify each SLRU page once.
 	 */
-	for (i = 0, headxid = xid;;)
+	headxid = xid;
+	i = 0;
+	for (;;)
 	{
 		int			pageno = TransactionIdToCTsPage(headxid);
 		int			j;
@@ -184,7 +186,7 @@ TransactionTreeSetCommitTsData(TransactionId xid, int nsubxids,
 							 pageno);
 
 		/* if we wrote out all subxids, we're done. */
-		if (j + 1 >= nsubxids)
+		if (j >= nsubxids)
 			break;
 
 		/*
@@ -192,7 +194,7 @@ TransactionTreeSetCommitTsData(TransactionId xid, int nsubxids,
 		 * just wrote.
 		 */
 		headxid = subxids[j];
-		i += j - i + 1;
+		i = j + 1;
 	}
 
 	/* update the cached value in shared memory */