Merge request got updated after merge to show no changes

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6005/diffs shows 0 changes, but the commit had one change that showed before the merge:

It looks like the merge commit was tagged at Thu Aug 25 19:12:33 2016.

The DB shows that the merge request was updated 10 seconds after that commit:

irb(main):007:0> mr.updated_at
=> Thu, 25 Aug 2016 19:12:42 UTC +00:00

The only way I can think that this happened was that the MR in the DB wasn't updated to the merged state when the PostReceive task ran.

/cc: @DouweM, @jschatz1