R1:7654df5d3b3f

Fix insufficient loading for processing Differential mail

Summary: Some actions (notably, `!accept`) require more information than we currently load.

Test Plan: Piped in some `!accept` mail using `bin/mail receive-test`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8844