Bypass post-checkout hook

Upstream msysgit add some code that drops a post-checkout hook into the repo, to manage Windows locking some stuff it needs to build Git. That's great, but it doesn't work with our release script because of paths and other nonsense. And, it doesn't apply to what we do in the release script anyway. This PR modifies our release script to temporarily disable it and then enable it when we are done.

cc @shiftkey