dirstate-item: add more logic to `from_p2`

This seem semantically better even if this tracking case seems to always comes
with clean_p2 for now.

Differential Revision: https://phab.mercurial-scm.org/D11372

mercurial/pure/parsers.py

@@ -286,7 +286,9 b' class DirstateItem(object):'
 
         Should only be set if a merge is in progress in the dirstate
         """
-        return self._wc_tracked and self._clean_p2
+        if not self._wc_tracked:
+            return False
+        return self._clean_p2 or (not self._p1_tracked and self._p2_tracked)
 
     @property
     def from_p2_removed(self):