Edit link of an exception to a ghost event should not create new event

Summary: Fixes T8382, Edit link of an exception to a ghost event should not create new event

Test Plan: Create recurring event, open ghost, edit, save, edit again. Should edit event created after first edit, not new event.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8382

Differential Revision: https://secure.phabricator.com/D13108