Allow "arc diff --draft" to create revisions as drafts more forcefully

Summary:
Depends on D18771. See PHI206. Currently, arc diff --draft only holds revisions in draft mode: it doesn't put them into draft mode if the install isn't configured to use draft mode.

Instead, make it a bit more forceful so that arc diff --draft can create into draft mode explicitly even if protoypes are off. This aligns with expection a little more clearly.

Test Plan: Ran arc diff --draft with prototypes off, got a revision held in draft mode.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18772