Allow "arc diff --plan-changes" to work with drafts enabled
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).
- Ran arc diff --plan-changes on master, got a "Changes Planned" revision instead of a validation error.
- Ran arc diff without --plan-changes, got a draft, verified that "Plan Changes" still doesn't appear in the action dropdown.
Maniphest Tasks: T13054
Differential Revision: https://secure.phabricator.com/D19067