Fix inadvertent forward dependency in task owner migration

Summary:
See <https://github.com/facebook/phabricator/issues/505>. When the status/event table moved, it broke this migration, which implicitly loads statuses by loading events.

Instead, access just the row we care about.

Test Plan: Used `--apply` to apply the new version of the patch.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D8162