Teach rsync transport about alternates.

Teach rsync transport about alternates.

For local operations and downloading and uploading via git aware protocols,
use of $GIT_OBJECT_DIRECTORY/info/alternates is recommended on the server
side for big projects that are derived from another one (like Linux kernel).

However, dumb protocols and rsync transport needs to resolve this on the
client end, which we did not bother doing until this week.

I noticed we use "rsync -z" but most of our payload is already compressed,
which was not quite right.  This commit also fixes it.

Signed-off-by: Junio C Hamano <junkio@cox.net>