R1:c666cb9f0bc7

Reduce the frequency of DOM scans to rebuild inlines when scrolling revisions

Summary:
Ref T13513. See PHI1734, which raises a concern about the performance of large revisions near the 100-change threshold.

Currently, `getInlines()` is called whenever the scroll position transitions between two changesets, and it performs a relatively complicated DOM scan to lift inlines out of the document.

This shows up as taking a small but nontrivial amount of time in Firefox profiles and should be safely memoizable.