delay PostUploadPack response until request is fully read

Git and Gitaly stream upload-pack response data as soon as it is
available. Some HTTP clients do not support the response being streamed
back while the server is still reading from the request body. To avoid
this, the request was buffered in to a temporary file before passing it
to Gitaly for handling. The buffer had a maximum size of 10MB, which is
now reached by requests to large repositories. These requests were then
truncated, causing Git to fail.

This commit fixes the problem by removing the request buffering and the
maximum size. Instead, the response is buffered in to a temporary file
until the request body is fully read, thus avoiding the problem of
streaming the request and the response simultaneously.