gitweb: Restructure projects list generation

gitweb: Restructure projects list generation

Extract filtering out forks (which is done if 'forks' feature is
enabled) into filter_forks_from_projects_list subroutine, and
searching projects (via projects search form, or via content tags)
into search_projects_list subroutine.

Both are now run _before_ displaying projects, and not while printing;
this allow to know upfront if there were any found projects.  Gitweb
now can and do print 'No such projects found' if user searches for
phrase which does not correspond to any project (any repository).
This also would allow splitting projects list into pages, if we so
desire.

Filtering out forks and marking repository (project) as having forks
is now consolidated into one subroutine (special case of handling
forks in git_get_projects_list only for $projects_list being file is
now removed).  Forks handling is also cleaned up and simplified.
$pr->{'forks'} now contains un-filled list of forks; we can now also
detect situation where the way for having forks is prepared, but there
are no forks yet.

Sorting projects got also refactored in a very straight way (just
moving code) into sort_projects_list subroutine.

The interaction between forks, content tags and searching is now made
more explicit: searching whether by tag, or via search form turns off
fork filtering (gitweb searches also forks, and will show all
results).  If 'ctags' feature is disabled, then searching by tag is
too.

The t9500 test now includes some basic test for 'forks' and 'ctags'
features; the t9502 includes test checking if gitweb correctly filters
out forks.

Generating list of projects by scanning given directory is now also a
bit simplified wrt. handling filtering; it is byproduct of extracting
filtering forks to separate subroutine.

While at it we now detect that there are no projects and respond with
"404 No projects found" also for 'project_index' and 'opml' actions.

Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>