fork off git-annex copy for transfers

This doesn't quite work, because canceling a transfer sends a signal
to git-annex, but not to rsync (etc).

Looked at making git-annex run in its own process group, which could then
be killed, and would kill child processes. But, rsync checks if it's
process group is the foreground process group and doesn't show progress if
not, and when git has run git-annex, if git-annex makes a new process
group, that is not the case. Also, if git has run git-annex, ctrl-c
wouldn't be propigated to it if it made a new process group.

So this seems like a blind alley, but recording it here just in case.

Assistant/TransferSlots.hs

@@ -11,7 +11,6 @@ import Control.Exception
 import Control.Concurrent
 
 import Common.Annex
-import Assistant.ThreadedMonad
 
 type TransferSlots = QSemN
 
@@ -29,13 +28,12 @@ newTransferSlots = newQSemN numSlots
 {- Waits until a transfer slot becomes available, and runs a transfer
  - action in the slot, in its own thread. Note that this thread is
  - subject to being killed when the transfer is canceled. -}
-inTransferSlot :: TransferSlots -> ThreadState -> Annex a -> IO ThreadId
-inTransferSlot s st a = do
+inTransferSlot :: TransferSlots -> IO () -> IO ThreadId
+inTransferSlot s a = do
 	waitQSemN s 1
-	forkIO $ bracket_ noop done run	
+	forkIO $ bracket_ noop done a
 	where
 		done = transferComplete s
-		run = unsafeRunThreadState st a
 
 {- Call when a transfer is complete. -}
 transferComplete :: TransferSlots -> IO ()