Add activity to deleted contacts showing they were deleted by merge

From 4.6 when 2 contacts are merged the remaining on gets an activity 'Contact Merged'. The source_contact_id is set to the person who did the merge, the target is the retained contact and there is no assignee.

This is a good thing (thank you JoAnne). However, it is not possible to search for 'Deleted contacts who were deleted as a result of a merge'.

I think there are 2 options

1) add a new activity Type 'Contact deleted in merge' with source being the logged in user and target being the deleted contact

2) make the deleted contact associated with the merge activity. My reservation about this is that it feels odd to make the deleted contact the source or assignee, but if we make them another target and they become undeleted there isn't a good way to search for them.

Out of scope but worth considering - it feels like adding an activity whenever a contact is deleted or undeleted would be a good thing.