separate source of content from the filename associated with the key when generating a key

This already made migrate's code a lot simpler.

Backend/SHA.hs

@@ -69,9 +69,10 @@ shaN size file = do
 		command = fromJust $ shaCommand size
 
 {- A key is a checksum of its contents. -}
-keyValue :: SHASize -> FilePath -> Annex (Maybe Key)
-keyValue size file = do
-	s <- shaN size file	
+keyValue :: SHASize -> KeySource -> Annex (Maybe Key)
+keyValue size source = do
+	let file = contentLocation source
+	s <- shaN size file
 	stat <- liftIO $ getFileStatus file
 	return $ Just $ stubKey
 		{ keyName = s
@@ -80,14 +81,14 @@ keyValue size file = do
 		}
 
 {- Extension preserving keys. -}
-keyValueE :: SHASize -> FilePath -> Annex (Maybe Key)
-keyValueE size file = keyValue size file >>= maybe (return Nothing) addE
+keyValueE :: SHASize -> KeySource -> Annex (Maybe Key)
+keyValueE size source = keyValue size source >>= maybe (return Nothing) addE
 	where
 		addE k = return $ Just $ k
 			{ keyName = keyName k ++ extension
 			, keyBackendName = shaNameE size
 			}
-		naiveextension = takeExtension file
+		naiveextension = takeExtension $ keyFilename source
 		extension
 			-- long or newline containing extensions are 
 			-- probably not really an extension

Backend/URL.hs

@@ -20,11 +20,11 @@ backends :: [Backend]
 backends = [backend]
 
 backend :: Backend
-backend = Backend {
-	name = "URL",
-	getKey = const (return Nothing),
-	fsckKey = Nothing
-}
+backend = Backend
+	{ name = "URL"
+	, getKey = const $ return Nothing
+	, fsckKey = Nothing
+	}
 
 fromUrl :: String -> Maybe Integer -> Key
 fromUrl url size = stubKey

Backend/WORM.hs

@@ -15,11 +15,11 @@ backends :: [Backend]
 backends = [backend]
 
 backend :: Backend
-backend = Backend {
-	name = "WORM",
-	getKey = keyValue,
-	fsckKey = Nothing
-}
+backend = Backend
+	{ name = "WORM"
+	, getKey = keyValue
+	, fsckKey = Nothing
+	}
 
 {- The key includes the file size, modification time, and the
  - basename of the filename.
@@ -28,11 +28,11 @@ backend = Backend {
  - while also allowing a file to be moved around while retaining the
  - same key.
  -}
-keyValue :: FilePath -> Annex (Maybe Key)
-keyValue file = do
-	stat <- liftIO $ getFileStatus file
+keyValue :: KeySource -> Annex (Maybe Key)
+keyValue source = do
+	stat <- liftIO $ getFileStatus $ contentLocation source
 	return $ Just Key {
-		keyName = takeFileName file,
+		keyName = takeFileName $ keyFilename source,
 		keyBackendName = name backend,
 		keySize = Just $ fromIntegral $ fileSize stat,
 		keyMtime = Just $ modificationTime stat