R1:43a8d8763df3

Update out-of-date API calls when rendering diffs inline in email

Summary: See PHI1745. This callsite for "ChangesetParser" was not properly updated for recent changes.

Test Plan:
- Set `metamta.differential.inline-patches` to 100.
- Created a new revision with a small (<100 line) diff, with at least one reviewer.
- Ran `bin/phd debug` and observed outbound mail queue with `bin/mail list-outbound`.
- Before: fatal when trying to generate the inline changes for mail.
- After: clean mail…