gitweb: Add a per-repository authorization hook.

gitweb: Add a per-repository authorization hook.

Add a configuration variable that can be used to specify an
arbitrary subroutine that will be called in the same situations
where $export_ok is checked, and its return value used
to decide whether the repository is to be shown.

This allows the user to implement custom authentication
schemes, for example by issuing a subrequest through mod_perl
and checking if Apache will authorize it.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>