This fixes the smart HTTP transport implementation receive pack service to send requests to `/git-receive-pack` instead of `/info/Refs?service=git-upload-pack`.
This prevents error "read/write on closed pipe".
Go's http.client::send() always closes req.Body, so if the first request attempt
is unsuccessful, any subsequent requests after calling the `CredentialsCallback`
will attempt to read/write on a closed pipe.