DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
We need to make sure merge_requests.source_project_id allows NULL values _before_ updating rows as otherwise this will lead to a NOT NULL constraint failing.
This adds various foreign keys and indexes to the "merge_requests" table as outlined in https://gitlab.com/gitlab-org/gitlab-ce/issues/31825. Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31825