Make stages index background migration more idempotent

lib/gitlab/background_migration/migrate_stage_index.rb

@@ -18,6 +18,7 @@ module Gitlab
                 WHERE ci_builds.stage_id = ci_stages.id
                 GROUP BY ci_builds.stage_idx ORDER BY COUNT(*) DESC LIMIT 1)
           WHERE ci_stages.id BETWEEN #{start_id.to_i} AND #{stop_id.to_i}
+            AND ci_stages.index IS NULL
         SQL
 
         ActiveRecord::Base.connection.execute(sql)