EE: Fix merge_requests.source_project_id migration
This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15496 so master
doesn't break (or stay broken) until the next sync.
This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15496 so master
doesn't break (or stay broken) until the next sync.