make Reduce.Render merge older into younger

for more efficient LatestMap.Merge

It isn't obvious that renderer.Render(rpt, dct) is younger than result,
but testing has shown this to be the case.

render/render.go

@@ -41,7 +41,7 @@ func MakeReduce(renderers ...Renderer) Renderer {
 func (r *Reduce) Render(rpt report.Report, dct Decorator) report.Nodes {
 	result := report.Nodes{}
 	for _, renderer := range *r {
-		result = result.Merge(renderer.Render(rpt, dct))
+		result = renderer.Render(rpt, dct).Merge(result)
 	}
 	return result
 }