implement VURL to URL migration

Just for completeness, someone might want to downgrade I guess?

Backend/VURL.hs

@@ -17,6 +17,7 @@ import Logs.EquivilantKeys
 import Backend.Variety
 import Backend.Hash (descChecksum)
 import Utility.Hash
+import Backend.VURL.Utilities
 
 backends :: [Backend]
 backends = [backendVURL]
@@ -69,7 +70,7 @@ backendVURL = Backend
 			, descIncrementalVerifier = descChecksum
 			} 
 	, canUpgradeKey = Nothing
-	, fastMigrate = Nothing
+	, fastMigrate = Just migrateFromVURLToURL
 	-- Even if a hash is recorded on initial download from the web and
 	-- is used to verify every subsequent transfer including other
 	-- downloads from the web, in a split-brain situation there

Backend/VURL/Utilities.hs

@@ -27,14 +27,20 @@ migrateFromURLToVURL oldkey newbackend _af inannex
 			Just ek -> do
 				setEquivilantKey newkey ek
 				return (Just newkey)
-	| otherwise = do
-		liftIO $ print ("migrateFromURL", inannex, fromKey keyVariety oldkey)
-		return Nothing
+	| otherwise = return Nothing
   where
 	-- Relies on the first hash being cryptographically secure, and the
 	-- default hash used by git-annex.
 	hashbackend = Prelude.head Backend.Hash.backends
 
+migrateFromVURLToURL :: Key -> Backend -> AssociatedFile -> Bool -> Annex (Maybe Key)
+migrateFromVURLToURL oldkey newbackend _af _
+	| fromKey keyVariety oldkey == VURLKey && backendVariety newbackend == URLKey =
+		return $ Just $ mkKey $ const $
+			(keyData oldkey)
+				{ keyVariety = URLKey }
+	| otherwise = return Nothing
+
 -- The Backend must use a cryptographically secure hash.
 generateEquivilantKey :: Backend -> RawFilePath -> Annex (Maybe Key)
 generateEquivilantKey b f =