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).