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.

class.c

@@ -1024,6 +1024,18 @@ include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
     struct rb_id_table *const klass_m_tbl = RCLASS_M_TBL(klass_origin);
     VALUE original_klass = klass;
 
+    if (klass_m_tbl) {
+        VALUE original_module = module;
+
+        while (module) {
+            if (klass_m_tbl == RCLASS_M_TBL(module))
+                return -1;
+            module = RCLASS_SUPER(module);
+        }
+
+        module = original_module;
+    }
+
     while (module) {
         int c_seen = FALSE;
 	int superclass_seen = FALSE;
@@ -1032,8 +1044,6 @@ include_modules_at(const VALUE klass, VALUE c, VALUE module, int search_super)
         if (klass == c) {
             c_seen = TRUE;
         }
-	if (klass_m_tbl && klass_m_tbl == RCLASS_M_TBL(module))
-	    return -1;
         if (klass_origin != c || search_super) {
             /* ignore if the module included already in superclasses for include,
              * ignore if the module included before origin class for prepend

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