exp/template/html: do not escape the RHS of assignments

In

  {{$x := . | foo}}
  {{$x}}

the first action is a variable assignment that contributes
nothing to the output while the first is a use that needs
to be escaped.

This CL fixes escapeAction to distinguish assignments from
interpolations and to only modify interpolations.

R=nigeltao, r
CC=golang-dev
https://golang.org/cl/5143048

src/pkg/exp/template/html/escape.go

@@ -153,6 +153,10 @@ func (e *escaper) escape(c context, n parse.Node) context {
 
 // escapeAction escapes an action template node.
 func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
+	if len(n.Pipe.Decl) != 0 {
+		// A local variable assignment, not an interpolation.
+		return c
+	}
 	c = nudge(c)
 	s := make([]string, 0, 3)
 	switch c.state {

src/pkg/exp/template/html/escape_test.go

@@ -68,10 +68,20 @@ func TestEscape(t *testing.T) {
 			"<Goodbye>!",
 		},
 		{
-			"overescaping",
+			"overescaping1",
 			"Hello, {{.C | html}}!",
 			"Hello, <Cincinatti>!",
 		},
+		{
+			"overescaping2",
+			"Hello, {{html .C}}!",
+			"Hello, <Cincinatti>!",
+		},
+		{
+			"overescaping3",
+			"{{with .C}}{{$msg := .}}Hello, {{$msg}}!{{end}}",
+			"Hello, <Cincinatti>!",
+		},
 		{
 			"assignment",
 			"{{if $x := .H}}{{$x}}{{end}}",