Correctly attach users to identities

Summary: This never worked.

Test Plan: Ran bin/repository rebuild-identities and viewed identity objects with currentEffectiveUserIDs and no longer got errors about attempting to attach null objects instead of PhabricatorUser objects.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19495