git-ls-remote: send no-cache header when fetching info/refs

Proxies should not cache this file as it can cause a client to end up with
a stale version, as reported here:

http://marc.theaimsgroup.com/?l=git&m=114407944125389

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>