dirstate-item: point out that `merged` is set only with p1_tracked

This is currently True, and we will use this fact to simplify the API in the
next commit.  However, we add this assertion first to validate that this is
True in the whole test-suite.

mercurial/pure/parsers.py

@@ -102,6 +102,7 b' class DirstateItem(object):'
             msg = b'`merged` argument incompatible with `clean_p1`/`clean_p2`'
             raise error.ProgrammingError(msg)
 
+        assert not (merged and not p1_tracked)
         self._wc_tracked = wc_tracked
         self._p1_tracked = p1_tracked
         self._p2_tracked = p2_tracked