branchcache: do not copy the `_dirty` flag

If the inherited branch cache is dirty, it will be written on disk, and the
super-set did not need to modify it, the on disk value for the subset will be
re-useable as is. So the super set does not needs to write the very same content
itself.

mercurial/branchmap.py

@@ -569,9 +569,6 b' class branchcache(_BaseBranchCache):'
             closednodes=set(self._closednodes),
             verify_node=self._verify_node,
         )
-        # we copy will likely schedule a write anyway, but that does not seems
-        # to hurt to overschedule
-        other._dirty = self._dirty
         # also copy information about the current verification state
         other._verifiedbranches = set(self._verifiedbranches)
         return other