dirstate: enforce holding the lock while doing any changes

This seems like a sensible sanity check. This already caught the issue in
largefile. This will catch more issue when we start using similar context
manager for operation other than the parent changes.

mercurial/dirstate.py

@@ -158,6 +158,9 b' class dirstate:'
         the incoherent dirstate won't be written when wlock is
         released.
         """
+        if repo.currentwlock() is None:
+            msg = b"changing parents without holding the wlock"
+            raise error.ProgrammingError(msg)
         if self._invalidated_context:
             msg = "trying to use an invalidated dirstate before it has reset"
             raise error.ProgrammingError(msg)