Promote instance identity to the upstream and pass it to commit hooks

Summary:
Fixes T7019. In a cluster environment, pushes currently fail because the commit hook can't identify the instance.

For web processes, the hostname identifies the instance -- but we don't have a hostname in the hook.

For CLI processes, the environment identifies the instance -- but we don't have an environment in the hook under SVN.

Promote the instance identifier into the upstream and pack/unpack it explicitly for hooks. This is probably not useful for anyone but us, but the amount of special-purpose code we're introducing is very small.

I poked at trying to do this in a more general way, but:

• We MUST know this BEFORE we run code, so the normal subclassing stuff is useless.
• I couldn't come up with any other parameter which might ever be useful to pass in.

Test Plan: Used git push to push code through proxied HTTP, got a clean push.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7019

Differential Revision: https://secure.phabricator.com/D11495