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/Reinject.hs

@@ -12,6 +12,7 @@ import Command
 import Logs.Location
 import Annex.Content
 import qualified Command.Fsck
+import qualified Backend
 
 def :: [Command]
 def = [command "reinject" (paramPair "SRC" "DEST") seek
@@ -33,16 +34,20 @@ start (src:dest:[])
 		next $ whenAnnexed (perform src) dest
 start _ = error "specify a src file and a dest file"
 
-perform :: FilePath -> FilePath -> (Key, Backend) -> CommandPerform
-perform src _dest (key, backend) =
+perform :: FilePath -> FilePath -> Key -> CommandPerform
+perform src dest key = do
 	{- Check the content before accepting it. -}
-	ifM (Command.Fsck.checkKeySizeOr reject key src
-		<&&> Command.Fsck.checkBackendOr reject backend key src)
-		( do
-			unlessM move $ error "mv failed!"
-			next $ cleanup key
-		, error "not reinjecting"
-		)
+	v <- Backend.getBackend dest key
+	case v of
+		Nothing -> stop
+		Just backend ->
+			ifM (Command.Fsck.checkKeySizeOr reject key src
+				<&&> Command.Fsck.checkBackendOr reject backend key src)
+				( do
+					unlessM move $ error "mv failed!"
+					next $ cleanup key
+				, error "not reinjecting"
+				)
   where
 	-- the file might be on a different filesystem,
 	-- so mv is used rather than simply calling