gitweb: Use File::Find::find in git_get_projects_list

gitweb: Use File::Find::find in git_get_projects_list

Earlier code to get list of projects when $projects_list is a
directory (e.g. when it is equal to $projectroot) had a hardcoded flat
(one level) list of directories.  Allow for projects to be in
subdirectories also for $projects_list being a directory by using
File::Find.

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