GitWeb source contains a special function that implements the
export_ok check, but validate_project still uses a separate copy
of essentially the same code.
This patch makes it use the dedicated function, thus ensuring
that all checks are done through a single code path.
Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com> Acked-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>