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 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