Update NEWS, mark test_merge_of_old_branch as known failing.

NEWS

@@ -2,6 +2,9 @@
 
  * Add py.typed to allow type checking. (David Caro)
 
+ * Add tests demonstrating a bug in the walker code.
+   (Doug Hellman)
+
 0.20.11	2020-10-30
 
  * Fix wheels build on Linux. (Ruslan Kuprieiev)

dulwich/tests/test_walk.py

@@ -23,6 +23,7 @@
 from itertools import (
     permutations,
     )
+from unittest import expectedFailure
 
 from dulwich.diff_tree import (
     CHANGE_MODIFY,
@@ -164,6 +165,7 @@ class WalkerTest(TestCase):
         self.assertWalkYields([c3, c2, c1], [c3.id])
         self.assertWalkYields([c2, c1], [c2.id])
 
+    @expectedFailure
     def test_merge_of_old_branch(self):
         # The commit on the branch was made at a time before any of
         # the commits on master, but it was merged into master after