make 'git clone' ask the remote only for objects it cares about

make 'git clone' ask the remote only for objects it cares about

Current behavior of 'git clone' when not using --mirror is to fetch
everything from the peer, and then filter out unwanted refs just before
writing them out to the cloned repository.  This may become highly
inefficient if the peer has an unusual ref namespace, or if it simply
has "remotes" refs of its own, and those locally unwanted refs are
connecting to a large set of objects which becomes unreferenced as soon
as they are fetched.

Let's filter out those unwanted refs from the peer _before_ asking it
what refs we want to fetch instead, which is the most logical thing to
do anyway.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>