--- a/mercurial/revlog.py	Wed Feb 21 11:53:30 2024 +0100
+++ b/mercurial/revlog.py	Thu Dec 21 17:38:04 2023 +0000
@@ -132,6 +132,7 @@
 # max size of inline data embedded into a revlog
 _maxinline = 131072
 
+
 # Flag processors for REVIDX_ELLIPSIS.
 def ellipsisreadprocessor(rl, text):
     return text, False
@@ -1577,7 +1578,6 @@
             ]
 
     def _loadindex(self, docket=None):
-
         new_header, mmapindexthreshold, force_nodemap = self._init_opts()
 
         if self.postfix is not None:
@@ -2345,6 +2345,12 @@
             return rustdagop.headrevs(self.index, revs)
         return dagop.headrevs(revs, self._uncheckedparentrevs)
 
+    def headrevsdiff(self, start, stop):
+        try:
+            return self.index.headrevsdiff(start, stop)
+        except AttributeError:
+            return dagop.headrevsdiff(self._uncheckedparentrevs, start, stop)
+
     def computephases(self, roots):
         return self.index.computephasesmapsets(roots)
 
@@ -2392,6 +2398,12 @@
 
         return [self.node(rev) for rev in revs]
 
+    def diffheads(self, start, stop):
+        """return the nodes that make up the difference between
+        heads of revs before `start` and heads of revs before `stop`"""
+        removed, added = self.headrevsdiff(start, stop)
+        return [self.node(r) for r in removed], [self.node(r) for r in added]
+
     def children(self, node):
         """find the children of a given node"""
         c = []