replace (Key, Backend) with Key

Only fsck and reinject and the test suite used the Backend, and they can
look it up as needed from the Key. This simplifies the code and also speeds
it up.

There is a small behavior change here. Before, all commands would warn when
acting on an annexed file with an unknown backend. Now, only fsck and
reinject show that warning.

Command/Direct.hs

@@ -47,7 +47,7 @@ perform = do
 	void $ liftIO clean
 	next cleanup
   where
-	go = whenAnnexed $ \f (k, _) -> do
+	go = whenAnnexed $ \f k -> do
 		r <- toDirectGen k f
 		case r of
 			Nothing -> noop