Fix completion tabbing.

I accidentally broke this in fb3682f5fa because
the variable gets reset before the slot is executed now.

See #189.

qutebrowser/utils/completer.py

@@ -179,7 +179,6 @@ class Completer(QObject):
         else:
             self._ignore_change = True
             self.change_completed_part.emit(data, False)
-            self._ignore_change = False
 
     @pyqtSlot(str, list, int)
     def on_update_completion(self, prefix, parts, cursor_part):
@@ -202,6 +201,7 @@ class Completer(QObject):
             cursor_part: The part the cursor is currently over.
         """
         if self._ignore_change:
+            self._ignore_change = False
             log.completion.debug("Ignoring completion update")
             return