Ref T6500, Duplicated tasks should be marked more visibly as duplicates

Summary: When merging tasks, the corresponding transaction on the merged task should be black, and the transaction on the ultimate task should be green.

Test Plan: Create two tasks, merge one into the other, merged task transaction is black, the surviving task should show a green transaction.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: chad, Korvin, epriestley

Maniphest Tasks: T6500

Differential Revision: https://secure.phabricator.com/D10827