R1:d8550c114d70

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…