Rename "arc diff --preview" to "arc diff --only"

Summary:
Experimental branch. Depends on D18739. Ref T2543.

To prepare for --draft, consolidate --preview and --only.

The old --preview is now arc diff --only.

The old --only is now arc diff --only --nounit --nolint.

I think this is more clear than other ways we could do it, since the difference between "draft" and "preview" seems less clear than the difference between "draft" and "only".

Test Plan: Ran arc diff --only and arc diff --only --nounit --nolint.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18740