Push to staging areas when running "arc diff"

Summary: Ref T8238. If a staging area is configured for a repository (see D13019), push a copy of changes to it after creating a diff.

Test Plan: Ran arc diff with various options, saw applicable changes get pushed to staging.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: cburroughs, epriestley

Maniphest Tasks: T8238

Differential Revision: https://secure.phabricator.com/D13020