http.postbuffer: allow full range of ssize_t values
Unfortunately, in order to push some large repos where a server does
not support chunked encoding, the http postbuffer must sometimes
exceed two gigabytes. On a 64-bit system, this is OK: we just malloc
a larger buffer.
This means that we need to use CURLOPT_POSTFIELDSIZE_LARGE to set the
buffer size.
Signed-off-by: David Turner <dturner@twosigma.com> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>