Diff: Handle deletions on replacements

Previously we assumed that if a line gets changed and the following
lines get deleted, the difflib generates two results for us. This is
wrong and this patch is the correction.

See #316

coalib/results/Diff.py

@@ -45,6 +45,8 @@ def from_string_arrays(cls, file_array_1, file_array_2):
                                        file_array_2[b_index_1])
                     result.add_lines(a_index_1+1,
                                      file_array_2[b_index_1+1:b_index_2])
+                    for index in range(a_index_1+2, a_index_2+1):
+                        result.delete_line(index)
 
         return result
 

coalib/tests/results/DiffTest.py

@@ -104,6 +104,11 @@ def test_from_string_arrays(self):
         self.uut = Diff.from_string_arrays(a, b)
         self.assertEqual(self.uut.apply(a), b)
 
+        a = ["first", "second", "third", "fourth"]
+        b = ["first_changed", "second_changed", "fourth"]
+        self.uut = Diff.from_string_arrays(a, b)
+        self.assertEqual(self.uut.apply(a), b)
+
 
 if __name__ == '__main__':
     unittest.main(verbosity=2)