R1:39b85c0be039

Use the changeset parse cache when rendering inline comments in mail

Summary:
Ref T13202. See PHI903 and PHI894. When a bot leaves 100 inline comments on the same file and the revision has a 30-member recipient list, we currently highlight the file 3000 times when building mail.

Instead, engage the parse cache so we highlight it once and reuse the cache 2,999 times.

Test Plan:
- Added debugging code to stop after mail generation and show cache hits/misses.
- Left a bunch of inlines in a file.