diff -r 5d68c4eedd66 -r a660d8a53267 mercurial/pure/parsers.py
--- a/mercurial/pure/parsers.py	Wed Sep 22 15:17:12 2021 +0200
+++ b/mercurial/pure/parsers.py	Wed Sep 22 17:46:29 2021 +0200
@@ -263,6 +263,24 @@
         self._size = None
         self._mtime = None
 
+    def drop_merge_data(self):
+        """remove all "merge-only" from a DirstateItem
+
+        This is to be call by the dirstatemap code when the second parent is dropped
+        """
+        if not (self.merged or self.from_p2):
+            return
+        self._p1_tracked = self.merged  # why is this not already properly set ?
+
+        self._merged = False
+        self._clean_p1 = False
+        self._clean_p2 = False
+        self._p2_tracked = False
+        self._possibly_dirty = True
+        self._mode = None
+        self._size = None
+        self._mtime = None
+
     @property
     def mode(self):
         return self.v1_mode()