Always return HTML in git_not_found

This allows us to give a nice 404 for e.g. archive.zip.

app/controllers/application_controller.rb

@@ -154,7 +154,7 @@ class ApplicationController < ActionController::Base
   end
 
   def git_not_found!
-    render "errors/git_not_found", layout: "errors", status: 404
+    render html: "errors/git_not_found", layout: "errors", status: 404
   end
 
   def method_missing(method_sym, *arguments, &block)