Remove "arc diff --only"
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.
Reviewed By: amckinley
Maniphest Tasks: T2543
Differential Revision: https://secure.phabricator.com/D18739