Use CustomFields in differential.createrevision

Summary:
Ref T2222. Ref T3886. Medium term goal is to remove DifferentialRevisionEditor.

This temporarily reduces a couple of pieces of functionality unique to the RevisionEditor, but I'm going to go clean those up in the next couple diffs.

Test Plan: Used arc diff --create to create several revisions with different data.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886, T2222

Differential Revision: https://secure.phabricator.com/D8452