Fix "arc diff --verbatim" when updating a revision

Summary: Fixes T4601. The "Differential Revision" field needs to be present in the "editable" version of the message so that `--verbatim` works correctly. Some day all of this might get rewritten to be a little easier to follow, maybe, but keep things working properly for now.

Test Plan: Used `arc diff`, `arc diff --edit`, `arc diff --verbatim`

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4601

Differential Revision: https://secure.phabricator.com/D8526