send-pack: take refspecs over stdin

send-pack: take refspecs over stdin

Pushing a large number of refs works over most transports,
because we implement send-pack as an internal function.
However, it can sometimes fail when pushing over http,
because we have to spawn "git send-pack --stateless-rpc" to
do the heavy lifting, and we pass each refspec on the
command line. This can cause us to overflow the OS limits on
the size of the command line for a large push.

We can solve this by giving send-pack a --stdin option and
using it from remote-curl.  We already dealt with this on
the fetch-pack side in 078b895 (fetch-pack: new --stdin
option to read refs from stdin, 2012-04-02). The stdin
option (and in particular, its use of packet-lines for
stateless-rpc input) is modeled after that solution.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>