process group killing

This seems to work pretty well.

Handled the process groups like this:

- git-annex processes started by the assistant for transfers are run in their
  own process groups.
- otherwise, rely on the shell to allocate a process group for git-annex

There is potentially a problem if some other program runs git-annex
directly (not using sh -c) The program and git-annex would then be in
the same process group. If that git-annex starts a transfer and it's
canceled, the program would also get killed. May or may not be a desired
result.

Also, the new updateTransferInfo probably closes a race where it was
possible for the thread id to not be recorded in the transfer info, if
the transfer info file from the transfer process is read first.

Assistant/DaemonStatus.hs

@@ -183,12 +183,17 @@ adjustTransfersSTM dstatus a = do
 	s <- takeTMVar dstatus
 	putTMVar dstatus $ s { currentTransfers = a (currentTransfers s) }
 
-{- Variant that does send notifications. -}
-adjustTransfers :: DaemonStatusHandle -> (TransferMap -> TransferMap) -> IO ()
-adjustTransfers dstatus a = 
+{- Updates a transfer's info. Preserves any transferTid value, which is not
+ - written to disk. -}
+updateTransferInfo :: DaemonStatusHandle -> Transfer -> TransferInfo -> IO ()
+updateTransferInfo dstatus t info =
 	notifyTransfer dstatus `after` modifyDaemonStatus_ dstatus go
 	where
-		go s = s { currentTransfers = a (currentTransfers s) }
+		go s = s { currentTransfers = update (currentTransfers s) }
+		update m = M.insertWith' merge t info m
+		merge new old = case transferTid old of
+			Nothing -> new
+			Just _ -> new { transferTid = transferTid old }
 
 {- Removes a transfer from the map, and returns its info. -}
 removeTransfer :: DaemonStatusHandle -> Transfer -> IO (Maybe TransferInfo)