dirstate: drop the last explicit `state` usage in the module

The internal detail of the `state` bytes is not longer exposed to the dirstate
layer.

Differential Revision: https://phab.mercurial-scm.org/D10977

mercurial/dirstate.py

@@ -536,7 +536,8 b' class dirstate(object):'
         if not self.in_merge:
             msg = _(b"setting %r to other parent only allowed in merges") % f
             raise error.Abort(msg)
-        if f in self and self[f] == b'n':
+        entry = self._map.get(f)
+        if entry is not None and entry.tracked:
             # merge-like
             self._addpath(f, merged=True)
         else: