Maniphest - fix bug updating tasks with blocked relationships

Summary: Ref T5604. Found this trying to open T5604 live. Basically this internal query needs the needSubscriberPHID set to true.

Test Plan: doing it live

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5604

Differential Revision: https://secure.phabricator.com/D10975