Fix a problem with recording image replacement, every other mock update

Summary:
This fixes a weird issue which currently doesn't have much impact on things, but starts to matter if we do the grid.

We're incorrectly initializing the form with replacesPHID as the previously replaced Image PHID. It is supposed to be the current File PHID.

Every other time, this is null and things work properly. On even updates (2, 4, 6, etc.), it's wrong and we don't record the replacement completely correctly.

Test Plan: Replaced images twice, saw three rows of thumb grid.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9541