This avoids "." and "..", as we already do, but also leaves
room for index-pack to store extra data in the quarantine
area (e.g., for passing back any analysis to be read by the
pre-receive hook).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>