Cross-project related merge request refs no longer work in RC2

Showing related merge requests was introduced in gitlab-org/gitlab-ce!1991 for RC1. It worked fine on GitLab.com at that point. After RC2 I can no longer get cross-project related merge requests to display. Same project references work fine.

cc/ @DouweM @rspeicher You were involved in the original implementation and have since done some stuff with the reference extractors. Can you take a look, please?