Remove "arc diff --only"

Summary:
Experimental branch. Depends on D18738. Ref T2543.

To prepare for adding --draft, clean up some of the flags behaviors.

--only currently means --preview --nolint --nounit. I'm going to:

• Remove --only.
• Rename --preview to --only, since I think --only is a better flag for this behavior than --preview.
• If you want the old --only, you can arc alias it to arc diff --only --nolint --nounit --browse or similar.

Test Plan: Grepped for 'only', only, etc.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18739