Fix visibility for DifferentialManiphestTasksField::readValueFromRevision

Summary: Ref T6822.

Test Plan: grep for ->readValueFromRevision(.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11364