Support receiving server capabilities

Support receiving server capabilities

This patch implements the client side of backward compatible upload-pack
protocol extension, <20051027141619.0e8029f2.vsu@altlinux.ru> by Sergey.

The updated server can append "server_capabilities" which is supposed
to be a string containing space separated features of the server, after
one of elements in the initial list of SHA1-refname line, hidden with
an embedded NUL.

After get_remote_heads(), check if the server supports the feature like

	if (server_supports("multi_ack"))
		do_something();

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>