Fix WAL replay of locking an updated tuple

We were resetting the tuple's HEAP_HOT_UPDATED flag as well as t_ctid on
WAL replay of a tuple-lock operation, which is incorrect when the tuple
is already updated.

Back-patch to 9.3.  The clearing of both header elements was there
previously, but since no update could be present on a tuple that was
being locked, it was harmless.

Bug reported by Peter Geoghegan and Greg Stark in
CAM3SWZTMQiCi5PV5OWHb+bYkUcnCk=O67w0cSswPvV7XfUcU5g@mail.gmail.com and
CAM-w4HPTOeMT4KP0OJK+mGgzgcTOtLRTvFZyvD0O4aH-7dxo3Q@mail.gmail.com
respectively; diagnosis by Andres Freund.

src/backend/access/heap/heapam.c

@@ -8016,11 +8016,19 @@ heap_xlog_lock(XLogRecPtr lsn, XLogRecord *record)
 
 	fix_infomask_from_infobits(xlrec->infobits_set, &htup->t_infomask,
 							   &htup->t_infomask2);
+
+	/*
+	 * Clear relevant update flags, but only if the modified infomask says
+	 * there's no update.
+	 */
+	if (HEAP_XMAX_IS_LOCKED_ONLY(htup->t_infomask))
+	{
 		HeapTupleHeaderClearHotUpdated(htup);
-	HeapTupleHeaderSetXmax(htup, xlrec->locking_xid);
-	HeapTupleHeaderSetCmax(htup, FirstCommandId, false);
 		/* Make sure there is no forward chain link in t_ctid */
 		htup->t_ctid = xlrec->target.tid;
+	}
+	HeapTupleHeaderSetXmax(htup, xlrec->locking_xid);
+	HeapTupleHeaderSetCmax(htup, FirstCommandId, false);
 	PageSetLSN(page, lsn);
 	MarkBufferDirty(buffer);
 	UnlockReleaseBuffer(buffer);