Fail gracefully when repositories have cycles

Fixes #451

makefile.shade

@@ -929,15 +929,31 @@ functions
             {
                 get
                 {
-                    if (Dependencies.Count > 0)
-                    {
-                        return 1 + Dependencies.Max(d => d.Order);
-                    }
-
-                    return 1;
+                    return GetOrder(new List<RepositoryInfo>(), this);
                 }
             }
 
+            private static int GetOrder(List<RepositoryInfo> visited, RepositoryInfo info)
+            {
+                if (visited.Contains(info))
+                {
+                    throw new Exception("A cyclic dependency between the following repositories has been detected: " +
+                        string.Join(" -> ", visited));
+                }
+
+                visited.Add(info);
+
+                var order = 0;
+                foreach (var dependency in info.Dependencies)
+                {
+                    order = Math.Max(order, GetOrder(visited, dependency));
+                }
+
+                visited.RemoveAt(visited.Count - 1);
+
+                return order + 1;
+            }
+
             public override string ToString()
             {
                 return Name;