qt TxEditor: fix extraneous "not able to RBF later" warning

Imported wallets, and HD wallets with "use change" disabled, would always
get the warning before.

electrum/gui/qt/confirm_tx_dialog.py

@@ -570,9 +570,12 @@ class TxEditor(WindowModalDialog):
             messages.append(_('This payment will be merged with another existing transaction.'))
         # warn if we use multiple change outputs
         num_change = sum(int(o.is_change) for o in self.tx.outputs())
+        num_ismine = sum(int(o.is_mine) for o in self.tx.outputs())
         if num_change > 1:
             messages.append(_('This transaction has {} change outputs.'.format(num_change)))
-        if num_change == 0:
+        # warn if there is no ismine output, as it might be problematic to RBF the tx later.
+        # (though RBF is still possible by adding new inputs, if the wallet has more utxos)
+        if num_ismine == 0:
             messages.append(_('Make sure you pay enough mining fees; you will not be able to bump the fee later.'))
 
         # TODO: warn if we send change back to input address