(stable) Update out-of-date API calls when rendering diffs inline in email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 generation.

Differential Revision: https://secure.phabricator.com/D21270