cmd/godoc: use virtual filesystem to implement -templates flag

R=golang-dev, gri, rsc
CC=golang-dev
https://golang.org/cl/5921045

src/cmd/godoc/godoc.go

@@ -491,14 +491,6 @@ var fmap = template.FuncMap{
 
 func readTemplate(name string) *template.Template {
 	path := "lib/godoc/" + name
-	if *templateDir != "" {
-		defaultpath := path
-		path = pathpkg.Join(*templateDir, name)
-		if _, err := fs.Stat(path); err != nil {
-			log.Print("readTemplate:", err)
-			path = defaultpath
-		}
-	}
 
 	// use underlying file system fs to read the template file
 	// (cannot use template ParseFile functions directly)

src/cmd/godoc/main.go

@@ -167,6 +167,9 @@ func main() {
 	if *zipfile == "" {
 		// use file system of underlying OS
 		fs.Bind("/", OS(*goroot), "/", bindReplace)
+		if *templateDir != "" {
+			fs.Bind("/lib/godoc", OS(*templateDir), "/", bindBefore)
+		}
 	} else {
 		// use file system specified via .zip file (path separator must be '/')
 		rc, err := zip.OpenReader(*zipfile)