text/template: further simplify building the vars list

Followup to https://golang.org/cl/197997

If you know the number of elements, you don't need append at all.
Either use append to grow, or allocate and index. Here we choose
number 2.

Change-Id: Ic58637231789640ff7b293ece04a95a8de7ccf8f
Reviewed-on: https://go-review.googlesource.com/c/go/+/198097Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/text/template/parse/node.go

@@ -187,9 +187,9 @@ func (p *PipeNode) CopyPipe() *PipeNode {
 	if p == nil {
 		return p
 	}
-	vars := make([]*VariableNode, 0, len(p.Decl))
-	for _, d := range p.Decl {
-		vars = append(vars, d.Copy().(*VariableNode))
+	vars := make([]*VariableNode, len(p.Decl))
+	for i, d := range p.Decl {
+		vars[i] = d.Copy().(*VariableNode)
 	}
 	n := p.tr.newPipeline(p.Pos, p.Line, vars)
 	n.IsAssign = p.IsAssign