CRM-13919 - Improved form rule to handle add mode differently.

----------------------------------------
* CRM-13919: Fatal error when user accidentally uses duplicate transaction # while recording a contrib.
  http://issues.civicrm.org/jira/browse/CRM-13919
Merge pull request #2181 from davecivicrm/CRM-13919a

CRM-13919 - Improved form rule to handle add mode differently.