receive-pack: run "gc --auto --quiet" and optionally "update-server-info"

receive-pack: run "gc --auto --quiet" and optionally "update-server-info"

Introduce two new configuration variables, receive.autogc (defaults to
true) and receive.updateserverinfo (defaults to false).  When these are
set, receive-pack runs "gc --auto --quiet" and "update-server-info"
respectively after it finishes receiving data from "git push" and updating
refs.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Nicolas Pitre <nico@fluxnic.net>