R1:2a5cf5e1b76c

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…