gitweb: Use single implementation of export_ok check.

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 <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>