Merge branch 'jk/http-server-protocol-versions'

Taking advantage of the CGI interface, http-backend has been updated to enable protocol v2 automatically when the other side asks for it. * jk/http-server-protocol-versions: docs/protocol-v2: point readers transport config discussion docs/git: discuss server-side config for GIT_PROTOCOL docs/http-backend: mention v2 protocol http-backend: handle HTTP_GIT_PROTOCOL CGI variable t5551: test v2-to-v0 http protocol fallback