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.