R1:0b6c0c135f84

Default new reviewers to "added", and don't overwrite reviewer states when updating

Summary:
Fixes two issues with Differential:

- New reviewers on initial diff were being created into a `null` state.
- The `"="` edge update was overwriting accepted/rejected statuses. This could maybe be more nuanced in the long run, but I've just made it update correctly for now.

Test Plan:
- Created and updated a revision, paying attention to reviewer statuses.

Reviewers: btrahan

Reviewed By: btrahan…