Merge d54d02b into merged_master (Elements PR ElementsProject#660)

Author: apoelstra

src/wallet/wallet.cpp

@@ -2804,6 +2804,11 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, int& nC
         }
     }
 
+    // Mark all un-returned change keys as used to reduce privacy loss
+    for (auto& changekey : vChangeKey) {
+        changekey->KeepKey();
+    }
+
     return true;
 }