coinchooser: tweak heuristic scoring.

transactions without any change now get better scores.
transactions with really small change get worse scores.

electrum/coinchooser.py

@@ -413,8 +413,13 @@ class CoinChooserPrivacy(CoinChooserRandom):
             tx, change_outputs = tx_from_buckets(buckets)
             change = sum(o.value for o in change_outputs)
             # Penalize change not roughly in output range
-            if change < min_change:
+            if change == 0:
+                pass  # no change is great!
+            elif change < min_change:
                 badness += (min_change - change) / (min_change + 10000)
+                # Penalize really small change; under 1 mBTC ~= using 1 more input
+                if change < COIN / 1000:
+                    badness += 1
             elif change > max_change:
                 badness += (change - max_change) / (max_change + 10000)
                 # Penalize large change; 5 BTC excess ~= using 1 more input