When $filter was empty, the path passed to check_export_ok would
contain an extra '/', which some implementations of export_auth_hook
are sensitive to.
It makes more sense to fix this here than to handle the special case
in each implementation of export_auth_hook.
Signed-off-by: Devin Doucette <devin@doucette.cc> Acked-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>