merge: mark mergestate as dirty when resolve changes _state

Correctness - no visible difference so far.

mercurial/merge.py

@@ -271,6 +271,7 b' class mergestate(object):'
         if r is None:
             # no real conflict
             del self._state[dfile]
+            self._dirty = True
         elif not r:
             self.mark(dfile, 'r')
         return r