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

Subscribers: aran, epriestley

Differential Revision: https://secure.phabricator.com/D8494