git: initialize hook app environment with paste as we do in other places

We want to use the whole paste / tg stack because it will do more
initialization for us - consistently, and in the right order.

This will allow a next change to avoid a problem in our custom beaker
initialization.

Going through paste might be a bit more heavy and slower than before, but it is
only run once for each git push.

Ultimately, we should probably get rid of the special hook entry point, and
just use kallithea-cli as the only entry point.