Fix the content ordering of the TOC for diffs

[khandelwal]

The table of contents in diff view were not sorting correctly. We saw:

appendix h, appendix i, supplement i, appendix j

when we should see:

appendix h, appendix i, appendix j, ..., supplement i

This fixes the sorting.

[khandelwal]

This code is now simpler, and ready for review.

[cmc333333]

Thanks!