Correct the generation of "<th />" IDs on left-hand-side of image changesets

Summary:
Fixes T7682. The left-hand-side "<th />" row did not generate with the correct ID.

(I couldn't reproduce the exact issue described in T7682, but hovering comments on either side now works properly for me.)

Test Plan: {F4962479}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7682

Differential Revision: https://secure.phabricator.com/D17926