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/Threads/TransferWatcher.hs

@@ -14,8 +14,6 @@ import Logs.Transfer
 import Utility.DirWatcher
 import Utility.Types.DirWatcher
 
-import Data.Map as M
-
 thisThread :: ThreadName
 thisThread = "TransferWatcher"
 
@@ -63,10 +61,7 @@ onAdd st dstatus file _ = case parseTransferFile file of
 				[ "transfer starting:"
 				, show t
 				]
-			adjustTransfers dstatus $
-				M.insertWith' merge t info
-		-- preseve transferTid, which is not written to disk
-		merge new old = new { transferTid = transferTid old }
+			updateTransferInfo dstatus t info
 
 {- Called when a transfer information file is removed. -}
 onDel :: Handler