create atom feed for commits

app/views/layouts/project.html.haml

@@ -5,6 +5,8 @@
       GitLab #{" - #{@project.name}" if @project && !@project.new_record?}
     = stylesheet_link_tag    "application"
     = javascript_include_tag "application"
+    - if current_page?(tree_project_path(@project)) || current_page?(project_commits_path(@project))
+      = auto_discovery_link_tag(:atom, project_commits_url(@project, :atom, :ref => @ref), :title => "Recent commits to #{@project.name}:#{@ref}")
     = csrf_meta_tags
     = javascript_tag do
       REQ_URI = "#{request.env["REQUEST_URI"]}";