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.

• Added debugging code to stop after mail generation and show cache hits/misses.
  • Left a bunch of inlines in a file.
  • Ran the worker with bin/worker execute --id ....
  • Before change: saw 4 comments x 2 recipients = 8 cache misses
  • After change: saw 8 cache hits (cache already filled from web UI rendering)
• Removed debugging code, ran worker to completion.
• Used bin/mail show-outbound --id ... --dump-html to inspect resulting mail, saw no functional differences.