R1:fcac85d80733

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…