Comment positioning bug in PRs with diff-gated patches

[vanessayuenn]

Steps to reproduce:

1. Make a PR with a large diff and a small diff (make sure the large one will be positioned above the small one)
2. Comment on the smaller diff
3. Open PR in atom
4. Observe that the comment is in the right spot
5. Expand the diff gated patch
6. Observe that the comment is now misplaced

Reproduced with this PR.