Do not let NGINX buffer Git HTTP requests

Before this change NGINX would convert a chunked HTTP POST (e.g. git push) into a HTTP 1.0 single large POST. This creates an unnecessary delay, and it creates unnecessary memory pressure on gitlab-git-http-server.

For the response ('proxy_buffering') I am less sure that NGINX 's buffering behavior is harmful, but it still makes more sense to me not to interfere with gitlab-git-http-server (and the Golang net/http server).