Use current ref for Files and Commits if available

Closes #1452

app/views/layouts/_project_menu.html.haml

@@ -5,9 +5,9 @@
   - if @project.repo_exists?
     - if can? current_user, :download_code, @project
       %li{class: current_controller?(:tree, :blob, :blame) ? 'active' : ''}
-        = link_to 'Files', project_tree_path(@project, @project.root_ref)
+        = link_to 'Files', project_tree_path(@project, @ref || @project.root_ref)
       %li{class: current_controller?(:commit, :commits, :compare, :repositories, :protected_branches) ? 'active' : ''}
-        = link_to "Commits", project_commits_path(@project, @project.root_ref)
+        = link_to "Commits", project_commits_path(@project, @ref || @project.root_ref)
       %li{class: tab_class(:network)}
         = link_to "Network", graph_project_path(@project)