R1:929f5f22acef

Fix an issue with adding or updating images in unified diff view

Summary: Fixes T8069.

Test Plan: Viewed a new image in unified diff view.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8069

Differential Revision: https://secure.phabricator.com/D13217