Use CustomField for differential.updaterevision

Summary: Ref T2222. Ref T3794. Medium term goal is to remove DifferentialRevisionEditor. This removes one of two callsites.

Test Plan: Used arc diff --edit to repeatedly update a revision, making changes to various fields.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3794, T2222

Differential Revision: https://secure.phabricator.com/D8451