gitweb: prepare git_get_projects_list for use outside 'forks'.

gitweb: prepare git_get_projects_list for use outside 'forks'.

Use of the filter option of git_get_projects_list is currently limited
to forks. It currently assumes the project belonging to the filter
directory was already validated to be visible in the project list.

To make it more generic add an optional argument to denote visibility
verification is still needed.

If there is a projects list file (GITWEB_LIST) only projects from
this list are returned anyway, so no more checks needed.

If there is no projects list file and the caller requests strict
checking (GITWEB_STRICT_EXPORT), do not jump directly to the
given directory but instead do a normal search and filter the
results instead.

The only effect of GITWEB_STRICT_EXPORT without GITWEB_LIST is to make
sure no project can be viewed without also be found starting from
project root. git_get_projects_list without this patch does not enforce
this but all callers only call it with a filter already checked this
way. With this parameter a caller can request this check if the filter
cannot be checked this way.

Signed-off-by: Bernhard R. Link <brlink@debian.org>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>