Branch graph returns 500 [5.2]

Created by: oinopion

After update to 5.2 graph display does not work

Started GET "/tiadmin/graph/master.json" for  at 2013-05-31 12:07:33 +0200
Processing by GraphController#show as JSON
  Parameters: {"project_id"=>"tiadmin", "id"=>"master"}
  Rendered graph/show.json.erb (3.2ms)
Completed 500 Internal Server Error in 133ms

ActionView::Template::Error (comparison of String with 0 failed):
  app/helpers/graph_helper.rb:7:in `>'
  app/helpers/graph_helper.rb:7:in `get_refs'
  app/views/graph/show.json.erb:16:in `block in _app_views_graph_show_json_erb___3120693441334642444_69482460'
  app/views/graph/show.json.erb:6:in `map'
  app/views/graph/show.json.erb:6:in `_app_views_graph_show_json_erb___3120693441334642444_69482460'