context: make sure __str__ works, also when there is no _changectx

Before, it could crash when trying to print the wrong kind of object at the
wrong time.

mercurial/context.py

@@ -709,7 +709,10 b' class basefilectx(object):'
             return False
 
     def __str__(self):
-        return "%s@%s" % (self.path(), self._changectx)
+        try:
+            return "%s@%s" % (self.path(), self._changectx)
+        except error.LookupError:
+            return "%s@???" % self.path()
 
     def __repr__(self):
         return "<%s %s>" % (type(self).__name__, str(self))