gitweb: support for no project list on gitweb front page

gitweb: support for no project list on gitweb front page

On very large sites like repo.or.cz (but maybe also git.debian.org,
git.kernel.org, etc.), it is desirable not to have the project list
on the front page since generating it is significant overhead and it
takes significant data transfer and load time for the user, who might
prefer to instead use the search form and possibly content tags to
navigate to the target project. A link to the full list of projects is
still available on the front page for users who wish to browse it. The
whole feature is turned off by default.

The patch introduces a new config variable $frontpage_no_project_list,
by default 0 keeping the current behavior; if set to 1, no project list
will be shown, but all projects will be still scanned if ctags are
enabled; if set to 2, no project will be shown and no projects will
be scanned while showing the front page. The compromise value of 1 is
useful for sites where project scan time is not an issue or which
use additional project list caching patches.

The patch furthermore modifies project_list action not to show the
index text, and introduces new default action frontpage which is by
default identical to old project_list action, but can be further
controlled by the $frontpage_no_project_list variable.

Signed-off-by: Petr Baudis <pasky@ucw.cz>
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>