Truncate the text passed to difflib where possible

This stops difflib from printing many lines which had no change in
them anyway.  It also avoids a bug in difflib which fails or hangs
when there are many trailing lines which are all identical.

--HG--
branch : trunk

doc/example/assertion/failure_demo.py

@@ -128,15 +128,33 @@ class TestSpecialisedExplanations(object):
     def test_eq_multiline_text(self):
         assert 'foo\nspam\nbar' == 'foo\neggs\nbar'
 
+    def test_eq_long_text(self):
+        a = '1'*100 + 'a' + '2'*100
+        b = '1'*100 + 'b' + '2'*100
+        assert a == b
+
+    def test_eq_long_text_multiline(self):
+        a = '1\n'*100 + 'a' + '2\n'*100
+        b = '1\n'*100 + 'b' + '2\n'*100
+        assert a == b
+
     def test_eq_list(self):
         assert [0, 1, 2] == [0, 1, 3]
 
+    def test_eq_list_long(self):
+        a = [0]*100 + [1] + [3]*100
+        b = [0]*100 + [2] + [3]*100
+        assert a == b
+
     def test_eq_dict(self):
         assert {'a': 0, 'b': 1} == {'a': 0, 'b': 2}
 
     def test_eq_set(self):
         assert set([0, 10, 11, 12]) == set([0, 20, 21])
 
+    def test_in_list(self):
+        assert 1 in [0, 2, 3, 4, 5]
+
 
 def globf(x):
     return x+1