text/template/parse: specify slice capacity in Pipenode.CopyPipe()

The required vars slice capacity is known so it can be specified before appending.

Change-Id: Ifa2fe97602e84198c4d01e5a1b0529f3f65f2df1
GitHub-Last-Rev: a0580df208a1d498968138d63508ae4e30df2ec5
GitHub-Pull-Request: golang/go#34613
Reviewed-on: https://go-review.googlesource.com/c/go/+/197997Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/text/template/parse/node.go

@@ -187,7 +187,7 @@ func (p *PipeNode) CopyPipe() *PipeNode {
 	if p == nil {
 		return p
 	}
-	var vars []*VariableNode
+	vars := make([]*VariableNode, 0, len(p.Decl))
 	for _, d := range p.Decl {
 		vars = append(vars, d.Copy().(*VariableNode))
 	}