diff --git a/mercurial/obsutil.py b/mercurial/obsutil.py
--- a/mercurial/obsutil.py
+++ b/mercurial/obsutil.py
@@ -397,12 +397,14 @@ def _cmpdiff(leftctx, rightctx):
 
     This is a first and basic implementation, with many shortcoming.
     """
-    diffopts = diffutil.diffallopts(leftctx.repo().ui, {'git': True})
+    # lefctx.repo() and rightctx.repo() are the same here
+    repo = leftctx.repo()
+    diffopts = diffutil.diffallopts(repo.ui, {'git': True})
     # Leftctx or right ctx might be filtered, so we need to use the contexts
     # with an unfiltered repository to safely compute the diff
-    leftunfi = leftctx._repo.unfiltered()[leftctx.rev()]
+    leftunfi = repo.unfiltered()[leftctx.rev()]
     leftdiff = leftunfi.diff(opts=diffopts)
-    rightunfi = rightctx._repo.unfiltered()[rightctx.rev()]
+    rightunfi = repo.unfiltered()[rightctx.rev()]
     rightdiff = rightunfi.diff(opts=diffopts)
 
     left, right = (0, 0)