R1:3c4322252513

Fix paging fatal with flagged objects

Summary: Fixes T13331. Just adds a generic `withIDs()` method to `PhabricatorFlagQuery`.

Test Plan: Flagged > 100 objects, observed fatal attempting to page. Next page loads as expected after fix.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T13331

Differential Revision: https://secure.phabricator.com/D20642