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