Error 500 loading merge requests: LocalJumpError: break from proc-closure

https://sentry.gitlab.net/gitlab/gitlabcom/issues/966118/

LocalJumpError: break from proc-closure
  projects/merge_requests_controller.rb:81:in `block (2 levels) in show'
    break render_404 unless @merge_request.diff_refs
  action_controller/metal/mime_responds.rb:203:in `respond_to'
    response.call if response
  projects/merge_requests_controller.rb:47:in `show'
    respond_to do |format|
  action_controller/metal/basic_implicit_render.rb:6:in `send_action'
    super.tap { default_render unless performed? }
  abstract_controller/base.rb:194:in `process_action'
    send_action(method_name, *args)
...
(124 additional frame(s) were not displayed)

LocalJumpError: break from proc-closure