Guarantee that fields copied from diffs persist on revisions

Summary:
Fixes T4636. Currently, we copy fields from the diff to the revision during the external effect phase, but there's no guarantee that we persist the object after this phase.

(In practice, when Herald rules trigger they cause the object to persist on this install, which is why we don't see this issue.)

Instead, move the field copies to the internal phase, where persistence is guaranteed.

Also consolidate some of the diff loading.

Test Plan: Ran arc diff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: mbishopim3, epriestley

Maniphest Tasks: T4636

Differential Revision: https://secure.phabricator.com/D8610