--- a/mercurial/pure/bdiff.py Mon Nov 16 16:38:57 2020 +0100
+++ b/mercurial/pure/bdiff.py Sat Nov 07 16:45:58 2020 -0800
@@ -51,7 +51,10 @@
shift += 1
r.append((a1, b1, l1 + shift))
prev = a2 + shift, b2 + shift, l2 - shift
- r.append(prev)
+
+ if prev is not None:
+ r.append(prev)
+
return r