dirstate-item: implement `merged` in a simpler way

We can simply use the underlying attribute.

Differential Revision: https://phab.mercurial-scm.org/D11367

mercurial/pure/parsers.py

@@ -276,7 +276,7 b' class DirstateItem(object):'
 
         Should only be set if a merge is in progress in the dirstate
         """
-        return self.v1_state() == b'm'
+        return self._wc_tracked and self._merged
 
     @property
     def from_p2(self):