R1:11c999413493

Allow "arc diff --plan-changes" to work with drafts enabled

Summary:
See PHI346. Ref T13054. If you have prototypes enabled on the server but use `master` / `stable` on the client and run `arc diff --plan-changes`, the transition is rejected because "Draft -> Changes Planned" isn't currently a legal transition.

Allow this transition if not coming from the web UI (to keep it out of the dropdown).

Test Plan:
- Ran `arc diff --plan-changes` on `master`, got a "Changes Planned" revision instead of a…