Add "rake gitlab:list_repos" task

doc/raketasks/list_repos.md

+# Listing repository directories
+
+You can print a list of all Git repositories on disk managed by
+GitLab with the following command:
+
+```
+# Omnibus
+sudo gitlab-rake gitlab:list_repos
+
+# Source
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:list_repos RAILS_ENV=production
+```
+
+If you only want to list projects with recent activity you can pass
+a date with the 'SINCE' environment variable.  The time you specify
+is parsed by the Rails [TimeZone#parse
+function](http://api.rubyonrails.org/classes/ActiveSupport/TimeZone.html#method-i-parse).
+
+```
+# Omnibus
+sudo gitlab-rake gitlab:list_repos SINCE='Sep 1 2015'
+
+# Source
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:list_repos RAILS_ENV=production SINCE='Sep 1 2015'
+```
+
+Note that the projects listed are NOT sorted by activity; they use
+the default ordering of the GitLab Rails application.

lib/tasks/gitlab/list_repos.rake

+namespace :gitlab do
+  task list_repos: :environment do
+    scope = Project
+    if ENV['SINCE']
+      date = Time.parse(ENV['SINCE'])
+      warn "Listing repositories with activity since #{date}"
+      project_ids = Project.where(['last_activity_at > ?', date]).pluck(:id)
+      scope = scope.where(id: project_ids)
+    end
+    scope.find_each do |project|
+      base = File.join(Gitlab.config.gitlab_shell.repos_path, project.path_with_namespace)
+      puts base + '.git'
+      puts base + '.wiki.git'
+    end
+  end
+end