Merge request ref not fetched in target repo?

https://sentry.gitlap.com/gitlab/gitlabcom/issues/4804/

The MR was created on 2015-07-23 and the error seems to indicate that the reference is not fetched in the target repo, even though we make sure the reference is fetched in the controller (with MergeRequest#ensure_ref_fetched)...

Rugged::OdbError: Object not found - no match for id (a92b10aef6e9e387c27771a3f1313f00e22dc0a7)
  lib/gitlab/metrics/instrumentation.rb:153:in `lookup'
    retval   = super
  app/models/repository.rb:783:in `can_be_merged?'
    their_commit = rugged.lookup(source_sha)
  app/models/merge_request.rb:264:in `check_if_can_be_merged'
    !broken? && project.repository.can_be_merged?(source_sha, target_branch)
  lib/gitlab/metrics/instrumentation.rb:153:in `check_if_can_be_merged'
    retval   = super
  app/models/merge_request.rb:294:in `mergeable?'
    check_if_can_be_merged
...
(150 additional frame(s) were not displayed)

ActionView::Template::Error: Object not found - no match for id (a92b10aef6e9e387c27771a3f1313f00e22dc0a7)
  lib/gitlab/metrics/instrumentation.rb:153:in `lookup'
    retval   = super
  app/models/repository.rb:783:in `can_be_merged?'
    their_commit = rugged.lookup(source_sha)
  app/models/merge_request.rb:264:in `check_if_can_be_merged'
    !broken? && project.repository.can_be_merged?(source_sha, target_branch)
  lib/gitlab/metrics/instrumentation.rb:153:in `check_if_can_be_merged'
    retval   = super
  app/models/merge_request.rb:294:in `mergeable?'
    check_if_can_be_merged
...
(150 additional frame(s) were not displayed)