Ask the wiki repo, not Gollum, if it's empty

We need to skip empty repositories when creating a backup. Before this
change, we were asking gollum-lib if the wiki contains any _pages_. Now
we ask gitlab_git if the repository contains _files_. This should
resolve gollum_lib Grit timeouts in the backup script.

lib/backup/repository.rb

@@ -30,7 +30,7 @@ module Backup
 
         if File.exists?(path_to_repo(wiki))
           print " * #{wiki.path_with_namespace} ... "
-          if wiki.empty?
+          if wiki.repository.empty?
             puts " [SKIPPED]".cyan
           else
             output, status = Gitlab::Popen.popen(%W(git --git-dir=#{path_to_repo(wiki)} bundle create #{path_to_bundle(wiki)} --all))