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
The private commit email is automatically generated in the format: id-username@noreply.HOSTNAME GitLab instance admins are able to change the HOSTNAME portion, that defaults to Gitlab's hostname, to whatever they prefer.