!15665 consistently 502s because it fetches every commit

It's hard to load !15665 (merged).

• Profile: http://profiler.gitlap.com/20180109/aa10c482-fa3d-47e1-a41f-05f374971cde.html.gz
• SQL: http://profiler.gitlap.com/20180109/aa10c482-fa3d-47e1-a41f-05f374971cde.txt.gz

We can see that 43% of the time is in Banzai::ReferenceParser::CommitParser#referenced_by, which should only load commits referenced in comments. However, it might also be loading commits referenced in system notes, which will (by definition) be all of them.