Update httpgzip package import path.

The github.com/shurcooL/go/gzip_file_server package has been overhauled
to have better style, and moved to import path github.com/shurcooL/httpgzip.

main.go

@@ -59,7 +59,6 @@ import (
 	"github.com/shurcooL/go-goon/bypass"
 	"github.com/shurcooL/go/analysis"
 	"github.com/shurcooL/go/gddo"
-	"github.com/shurcooL/go/gzip_file_server"
 	"github.com/shurcooL/go/httpstoppable"
 	"github.com/shurcooL/go/open"
 	"github.com/shurcooL/go/pipeutil"
@@ -68,6 +67,7 @@ import (
 	"github.com/shurcooL/go/reflectsource"
 	"github.com/shurcooL/go/trim"
 	"github.com/shurcooL/httpfs/union"
+	"github.com/shurcooL/httpgzip"
 	"github.com/shurcooL/markdownfmt/markdown"
 	"github.com/shurcooL/octicons"
 	"golang.org/x/net/websocket"
@@ -6450,7 +6450,7 @@ func initHttpHandlers() {
 		assets := union.New(map[string]http.FileSystem{
 			"/octicons": octicons.Assets,
 		})
-		fileServer := gzip_file_server.New(assets)
+		fileServer := httpgzip.FileServer(assets, httpgzip.FileServerOptions{ServeError: httpgzip.Detailed})
 		http.Handle("/assets/", http.StripPrefix("/assets", fileServer))
 	}
 	http.HandleFunc("/close", func(w http.ResponseWriter, req *http.Request) {