Maniphest - prevent uneditable tasks from being able to be closed as duplicates

Summary:
Fixes T7923.

Prevent the user from finding tasks that they can't edit in merge workflows. Also ensure that we query properly on final merge action just in case.

Test Plan: Tried to find a task I couldn't edit in various searches under the "merge" dialogue and couldn't find the task. Removed this big of code and tried to merge in a task and after hitting "merge" observed the page reloaded with no task merged in.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7923

Differential Revision: https://secure.phabricator.com/D12872