Fix transient fork_spec spec failure

Example: https://gitlab.com/gitlab-org/gitlab-ce/pipelines/8061800

 1) forked project import can access the MR
     Failure/Error: expect(project.merge_requests.first.ensure_ref_fetched.first).to include('refs/merge-requests/1/head')

     NoMethodError:
       undefined method `ensure_ref_fetched' for nil:NilClass
     # ./spec/lib/gitlab/import_export/fork_spec.rb:47:in `block (2 levels) in <top (required)>'

Looks like it's mySQL only.

Yes, I'm seeing this one a lot.

assigned to @jameslopez

mentioned in merge request !11121 (merged)

@jameslopez Thanks for tackling this one, it seems to be almost consistently failing now. :(

I noticed :(

closed via merge request !11121 (merged)

mentioned in commit 0e13c0a9

mentioned in merge request !14785