Configures nginx to rewrite all requests to cgit, except for those
retrieving static files, to `/depot/`.
In combination with the previous commits that apply patches to cgit
itself, this effectively means that the depot is rendered on the site
root.
This is pretty cool: It lets people do stuff like `git clone
https://git.tazj.in` and get the depot!
Apart from the fact that TLS certificate provisioning is very wonky,
it seems to be working now.
AFAICT the L7 LBs still don't support path rewriting, which means that
this is likely not the final configuration and it will move behind
nginx instead.
