When jobs share gates, Zuul has a nice feature which is to creates
reference of the Change in every involved repositories. That was sadly
undocumented.
Reference:
James E. Blair on openstack-infra mailing list:
http://lists.openstack.org/pipermail/openstack-infra/2014-January/000605.html
Change-Id: Ia3bd640f6a90e12192af1686547a5f4146192dbe