support prompt in message serialization

That seems to be the last thing needed for message serialization.
Although it's only used in the assistant currently, so hard to tell if I
forgot something.

At this point, it should be possible to start using transferkeys
when performing transfers, which will allow killing a transferkeys
process if a transfer times out or stalls. But that's for another day.

This commit was sponsored by Ethan Aubin.

Command/TransferKeys.hs

@@ -41,8 +41,9 @@ start :: CommandStart
 start = do
 	enableInteractiveBranchAccess
 	(readh, writeh) <- liftIO dupIoHandles
-	Annex.setOutput $ SerializedOutput $
-		hPutStrLn writeh . show . TransferOutput
+	Annex.setOutput $ SerializedOutput
+		(hPutStrLn writeh . show . TransferOutput)
+		(readMaybe <$> hGetLine readh)
 	runRequests readh writeh runner
 	stop
   where
@@ -101,7 +102,7 @@ runRequests readh writeh a = go Nothing Nothing
 		hPutStrLn writeh $ show $ TransferResult b
 		hFlush writeh
 
--- FIXME this is bad when used with inAnnex
+-- | Send a request to this command to perform a transfer.
 sendRequest :: Transfer -> TransferInfo -> Handle -> IO ()
 sendRequest t tinfo h = hPutStrLn h $ show $ TransferRequest
 	(transferDirection t)
@@ -109,6 +110,9 @@ sendRequest t tinfo h = hPutStrLn h $ show $ TransferRequest
 	(keyData (transferKey t))
 	(associatedFile tinfo)
 
+sendSerializedOutputResponse :: Handle -> SerializedOutputResponse -> IO ()
+sendSerializedOutputResponse h sor = hPutStrLn h $ show sor
+
 -- | Read a response from this command.
 --
 -- Before the final response, this will return whatever SerializedOutput