Toggle revision "shouldBroadcast" correctly when "--draft" is used with prototypes off

Summary:
See PHI573. Ref T13120. Drafts were recently changed so that "draft" and "broadcast" are separate flags, and you can have non-broadcasting revisions in states other than "draft" if builds fail on a draft or you abandon a draft.

However, when draft mode is entered with arc diff --draft and you have prototypes off, this flag wasn't being set correctly.

Test Plan: Disabled prototypes, created a revision with arc diff --draft, observed that draft.broadcast is now correctly false.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13120

Differential Revision: https://secure.phabricator.com/D19360