Add reference to the correct branch.

sometimes `@ref` will be `nil`.
this makes sure that the correct branch is passed in.

app/views/layouts/header/_default.html.haml

@@ -39,4 +39,4 @@
 = render 'shared/outdated_browser'
 -if defined?(@project)
   :javascript
-    var findFileURL = '#{namespace_project_find_file_path(@project.namespace, @project, @ref)}';
\ No newline at end of file
+    var findFileURL = '#{namespace_project_find_file_path(@project.namespace, @project, @ref || @project.repository.root_ref)}';
\ No newline at end of file