Separate "Revision" and "Diff" fields in Differential

Summary:
Fixes T5138. Some of the "revision" properties are really "diff" properties, but we only show the properties for the most recent / current diff.

• Immediately, this makes it hard or impossible to review, e.g., lint/unit results for older diffs.
• Longer-term, these limits will become more problematic with more data on diffs after Harbormaster.

Instead, separate "revision" from "diff" properties.

(In the long term, it might make sense to show more diffs in this panel -- e.g., tabs for the 8 most recent updates or something -- but I went with the simplest approach for now since I don't have a clean way to deal with 100-update revisions offhand.)

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: cburroughs, epriestley

Maniphest Tasks: T5138

Differential Revision: https://secure.phabricator.com/D13282