avoid using openFile when withFile can be used

Potentially fixes some FD leak if an action on an opened file handle fails
for some reason. There have been some hard to reproduce reports of
git-annex leaking FDs, and this may solve them.

Remote/Bup.hs

@@ -145,9 +145,8 @@ storeEncrypted r buprepo (cipher, enck) k _p =
 retrieve :: BupRepo -> Key -> AssociatedFile -> FilePath -> MeterUpdate -> Annex Bool
 retrieve buprepo k _f d _p = do
 	let params = bupParams "join" buprepo [Param $ bupRef k]
-	liftIO $ catchBoolIO $ do
-		tofile <- openFile d WriteMode
-		pipeBup params Nothing (Just tofile)
+	liftIO $ catchBoolIO $ withFIle d WriteMode $
+		pipeBup params Nothing . Just
 
 retrieveCheap :: BupRepo -> Key -> FilePath -> Annex Bool
 retrieveCheap _ _ _ = return False