separate rake task for migration and persistent one

lib/tasks/migrate/migrate_mr.rake

+# This taks will reload commits/diff for all merge requests
+desc "GITLAB | Migrate Merge Requests"
+task migrate_merge_requests: :environment do
+  MergeRequest.find_each(batch_size: 20) do |mr|
+    mr.st_commits = []
+    mr.save
+    mr.reload_code
+    print '.'
+  end
+end
+