qt confirm_tx_dialog: fix wallet.get_candidates_for_batching

I don't understand what the "coins not used" comment meant here.
It was added in the change away from the old config.WALLET_BATCH_RBF option
(ab14c3e138).

The `coins` param *is used* in wallet.get_candidates_for_batching.
Without setting that, the returned set of candidates was restricted to
only base txs that had a large enough change output to cover *all* the newly added outputs.
Instead, it is desirable to allow adding new inputs.

electrum/gui/qt/send_tab.py

@@ -327,7 +327,14 @@ class SendTab(QWidget, MessageBoxMixin, Logger):
         is_max = any(parse_max_spend(outval) for outval in output_values)
         output_value = '!' if is_max else sum(output_values)
 
-        candidates = self.wallet.get_candidates_for_batching(outputs, []) # coins not used
+        # To find batching candidates, we need to know our available UTXOs.
+        # Ideally should use same set of coins make_tx() will use.
+        # note: - prone to races: coins set might change due to new txs between now and make_tx() call
+        #       - make_tx() might pass different params to get_coins()
+        #         - to mitigate, we prefer to be more restrictive. hence confirmed_only=True
+        coins_conservative = get_coins(nonlocal_only=True, confirmed_only=True)
+        candidates = self.wallet.get_candidates_for_batching(outputs, coins=coins_conservative)
+
         tx, is_preview = self.window.confirm_tx_dialog(make_tx, output_value, batching_candidates=candidates)
         if tx is None:
             # user cancelled