Maniphest - upgrade merging to real transactions

Summary: see title. Ref T5875.

Test Plan: Merged one task into another task - verified transactions on both tasks. Merged two tasks into another task - verified transactions on all three tasks. Checked out my feed and saw MERGE_INTO stories and MERGE_FROM stories.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5875

Differential Revision: https://secure.phabricator.com/D10427