Add "arc diff --draft"

Summary:
Experimental branch. Ref T2543. Depends on D18742.

Add an "arc diff --draft" flag which holds revisions as drafts indefinitely.

Test Plan: Ran "arc diff --draft" when creating; ran "arc diff --draft" to try to update a revision and got a failure.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18743