merge contacts fails on external ID

When merging two contacts, if the duplicate contact has an External ID value, the merge breaks. Reproducible on sandbox:
http://drupal.demo.civicrm.org/civicrm/contact/merge?reset=1&cid=156&oid=157

Select all and merge.

The merge script uses sql UPDATE to transfer the value from record 1 to 2. But a unique value constraint on external_identifier prohibits transfer of that value to the retained record.

UPDATE civicrm_contact SET contact_type = 'Individual' , external_identifier = 'jd1' WHERE ( civicrm_contact.id = 156 )
Database Error Code: Duplicate entry 'jd2' for key 2, 1062

maybe the value can be stored in a temp var until the dup contact is deleted, then transferred to the retained record.