Allow curl to rewind the RPC read buffer

Allow curl to rewind the RPC read buffer

When using multi-pass authentication methods, the curl library may need
to rewind the read buffers used for providing data to HTTP POST, if data
has been output before a 401 error is received.

This is needed only when the first request (when the multi-pass
authentication method isn't initialized and hasn't received its challenge
yet) for a certain curl session is a chunked HTTP POST.

As long as the current rpc read buffer is the first one, we're able to
rewind without need for additional buffering.

The curl library currently starts sending data without waiting for a
response to the Expect: 100-continue header, due to a bug in curl that
exists up to curl version 7.19.7.

If the HTTP server doesn't handle Expect: 100-continue headers properly
(e.g. Lighttpd), the library has to start sending data without knowing
if the request will be successfully authenticated. In this case, this
rewinding solution is not sufficient - the whole request will be sent
before the 401 error is received.

Signed-off-by: Martin Storsjo <martin@martin.st>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>