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.

Test.hs

@@ -712,7 +712,7 @@ test_unused env = intmpclonerepoInDirect env $ do
 			(sort expectedkeys) (sort unusedkeys)
 	findkey f = do
 		r <- Backend.lookupFile f
-		return $ fst $ fromJust r
+		return $ fromJust r
 
 test_describe :: TestEnv -> Assertion
 test_describe env = intmpclonerepo env $ do
@@ -1233,7 +1233,7 @@ test_crypto env = do
 			(c,k) <- annexeval $ do
 				uuid <- Remote.nameToUUID "foo"
 				rs <- Logs.Remote.readRemoteLog
-				Just (k,_) <- Backend.lookupFile annexedfile
+				Just k <- Backend.lookupFile annexedfile
 				return (fromJust $ M.lookup uuid rs, k)
 			let key = if scheme `elem` ["hybrid","pubkey"]
 					then Just $ Utility.Gpg.KeyIds [Utility.Gpg.testKeyId]
@@ -1500,7 +1500,7 @@ checklocationlog f expected = do
 	thisuuid <- annexeval Annex.UUID.getUUID
 	r <- annexeval $ Backend.lookupFile f
 	case r of
-		Just (k, _) -> do
+		Just k -> do
 			uuids <- annexeval $ Remote.keyLocations k
 			assertEqual ("bad content in location log for " ++ f ++ " key " ++ Types.Key.key2file k ++ " uuid " ++ show thisuuid)
 				expected (thisuuid `elem` uuids)
@@ -1508,9 +1508,9 @@ checklocationlog f expected = do
 
 checkbackend :: FilePath -> Types.Backend -> Assertion
 checkbackend file expected = do
-	r <- annexeval $ Backend.lookupFile file
-	let b = snd $ fromJust r
-	assertEqual ("backend for " ++ file) expected b
+	b <- annexeval $ maybe (return Nothing) (Backend.getBackend file) 
+		=<< Backend.lookupFile file
+	assertEqual ("backend for " ++ file) (Just expected) b
 
 inlocationlog :: FilePath -> Assertion
 inlocationlog f = checklocationlog f True