Make a cyclic prepend not modify ancestors for the receiver

Check for cyclic prepend before making any changes.  This requires
scanning the module ancestor chain twice, but in general modules
do not have large numbers of ancestors.

test/ruby/test_module.rb

@@ -478,6 +478,13 @@ class TestModule < Test::Unit::TestCase
     assert_raise(ArgumentError) { Module.new { include } }
   end
 
+  def test_prepend_self
+    m = Module.new
+    assert_equal([m], m.ancestors)
+    m.prepend(m) rescue nil
+    assert_equal([m], m.ancestors)
+  end
+
   def test_prepend_works_with_duped_classes
     m = Module.new
     a = Class.new do