cmd/godoc: initialize filesystem and metadata for -url

Unlike when using -http, godoc -url didn't initialize the "filesystem"
and metadata that are used when generating responses. This CL adds this
initialization, so that -url provides the same results as an HTTP
request when using -http.

Fixes #4335.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/6817075

src/cmd/godoc/main.go

@@ -225,6 +225,8 @@ func main() {
 	// Print content that would be served at the URL *urlFlag.
 	if *urlFlag != "" {
 		registerPublicHandlers(http.DefaultServeMux)
+		initFSTree()
+		updateMetadata()
 		// Try up to 10 fetches, following redirects.
 		urlstr := *urlFlag
 		for i := 0; i < 10; i++ {