pgstattuple: Use SnapshotDirty, not SnapshotNow.

Tuples belonging to uncommitted transactions should not be
counted as dead.

This is arguably a bug fix that should be back-patched, but
as no one ever noticed until it came time to try to get rid
of SnapshotNow, I'm only doing this in master for now.

contrib/pgstattuple/pgstattuple.c

@@ -278,9 +278,11 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)
 	Buffer		buffer;
 	pgstattuple_type stat = {0};
 	BufferAccessStrategy bstrategy;
+	SnapshotData SnapshotDirty;
 
 	/* Disable syncscan because we assume we scan from block zero upwards */
 	scan = heap_beginscan_strat(rel, SnapshotAny, 0, NULL, true, false);
+	InitDirtySnapshot(SnapshotDirty);
 
 	nblocks = scan->rs_nblocks; /* # blocks to be scanned */
 
@@ -296,7 +298,7 @@ pgstat_heap(Relation rel, FunctionCallInfo fcinfo)
 		/* must hold a buffer lock to call HeapTupleSatisfiesVisibility */
 		LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);
 
-		if (HeapTupleSatisfiesVisibility(tuple, SnapshotNow, scan->rs_cbuf))
+		if (HeapTupleSatisfiesVisibility(tuple, &SnapshotDirty, scan->rs_cbuf))
 		{
 			stat.tuple_len += tuple->t_len;
 			stat.tuple_count++;