godoc: update metadata upon launch

Without this change it's possible to launch godoc,
immediately GET /, and see a directory listing instead of root.html

R=gri
CC=golang-dev
https://golang.org/cl/5575054

src/cmd/godoc/main.go

@@ -337,6 +337,8 @@ func main() {
 			}()
 		}
 
+		// Immediately update metadata.
+		updateMetadata()
 		// Periodically refresh metadata.
 		go refreshMetadataLoop()