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/Migrate.hs

@@ -25,15 +25,19 @@ def = [notDirect $
 seek :: CommandSeek
 seek = withFilesInGit $ whenAnnexed start
 
-start :: FilePath -> (Key, Backend) -> CommandStart
-start file (key, oldbackend) = do
-	exists <- inAnnex key
-	newbackend <- choosebackend =<< chooseBackend file
-	if (newbackend /= oldbackend || upgradableKey oldbackend key) && exists
-		then do
-			showStart "migrate" file
-			next $ perform file key oldbackend newbackend
-		else stop
+start :: FilePath -> Key -> CommandStart
+start file key = do
+	v <- Backend.getBackend file key
+	case v of
+		Nothing -> stop
+		Just oldbackend -> do
+			exists <- inAnnex key
+			newbackend <- choosebackend =<< chooseBackend file
+			if (newbackend /= oldbackend || upgradableKey oldbackend key) && exists
+				then do
+					showStart "migrate" file
+					next $ perform file key oldbackend newbackend
+				else stop
   where
 	choosebackend Nothing = Prelude.head <$> orderedList
 	choosebackend (Just backend) = return backend