R1:95a5b41b0ba2

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…