Prevent infinite recurse when 2 groups are members of each other

Tries to solve issue already reported:
theforeman#51

lib/ldap_fluff/ad_member_service.rb

@@ -20,7 +20,7 @@ def _groups_from_ldap_data(payload)
     data = []
     if !payload.nil?
       first_level  = payload[:memberof]
-      total_groups = _walk_group_ancestry(first_level)
+      total_groups, b = _walk_group_ancestry(first_level, first_level)
       data         = (get_groups(first_level + total_groups)).uniq
     end
     data
@@ -32,12 +32,15 @@ def _walk_group_ancestry(group_dns = [])
     group_dns.each do |group_dn|
       search = @ldap.search(:base => group_dn, :scope => Net::LDAP::SearchScope_BaseObject, :attributes => ['memberof'])
       if !search.nil? && !search.first.nil?
-        group = search.first
-        set  += _walk_group_ancestry(group[:memberof])
-        set  += group[:memberof]
+        groups = search.first[:memberof] - known_groups
+        known_groups += groups
+        next_level, new_known_groups = _walk_group_ancestry(groups, known_groups)
+        set += next_level
+        set += groups
+        known_groups += next_level
       end
     end
-    set
+    return [set, known_groups]
   end
 
   def class_filter