assistant: On Linux, the expensive transfer scan is run niced.

This is a compromise. I would like to nice every thread except for the
webapp thread, but it's not practical to do so. That would need every
thread to run as a bound thread, which could add significant overhead.
And any forkIO would escape the nice level.

Assistant/Threads/TransferScanner.hs

@@ -24,6 +24,7 @@ import qualified Remote
 import qualified Types.Remote as Remote
 import Utility.ThreadScheduler
 import Utility.NotificationBroadcaster
+import Utility.Batch
 import qualified Git.LsFiles as LsFiles
 import qualified Backend
 import Annex.Content
@@ -114,7 +115,7 @@ failedTransferScan r = do
  - since we need to look at the locations of all keys anyway.
  -}
 expensiveScan :: UrlRenderer -> [Remote] -> Assistant ()
-expensiveScan urlrenderer rs = unless onlyweb $ do
+expensiveScan urlrenderer rs = unless onlyweb $ batch <~> do
 	debug ["starting scan of", show visiblers]
 
 	unwantedrs <- liftAnnex $ S.fromList