handle git-credential prompt in -J mode

If git-credential has it cached and does not prompt, this will
unfortunately result in a brief flicker, as the displayed console
regions are hidden while running it and then re-displayed. Better than a
corrupted display.

Actually, I tried it and don't see a visible flicker, so probably only
over a slow ssh will it be apparent.

Utility/Url.hs

@@ -510,7 +510,7 @@ downloadConduit meterupdate req file uo =
 		case r of
 			Right () -> signalsuccess True
 			Left e -> do
-				signalsuccess False
+				() <- signalsuccess False
 				throwM e
 	
 {- Sinks a Response's body to a file. The file can either be opened in