Allow configuration of a "staging area" for each repository

Summary:
Ref T8238. This allows configuration of a "staging area" for Git repositories, which is the URI to some Git repository (possibly the same repository).

If a staging area is configured, arc will push a copy of anything it creates a diff for there (see next revision). This primarily makes handoff to build systems easier.

This is a bit leaky and I intend for it to eventually be positioned as a less-preferred solution, but from the perspective of build systems it's the same as the real (virtual ref) solution that I want to build.

Test Plan: Ran arc diff with various flags, saw appropriate changes copied into the staging area. See also discussion in T8238.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: cburroughs, epriestley

Maniphest Tasks: T8238

Differential Revision: https://secure.phabricator.com/D13019