remove read of the heads

and one tail

Removed head from Utility.PartialPrelude in order to avoid the build
warning with recent ghc versions as well.

Backend/VURL/Utilities.hs

@@ -31,7 +31,8 @@ migrateFromURLToVURL oldkey newbackend _af inannex
   where
 	-- Relies on the first hash being cryptographically secure, and the
 	-- default hash used by git-annex.
-	hashbackend = Prelude.head Backend.Hash.backends
+	hashbackend = fromMaybe (error "internal") $ 
+		headMaybe Backend.Hash.backends
 
 migrateFromVURLToURL :: Key -> Backend -> AssociatedFile -> Bool -> Annex (Maybe Key)
 migrateFromVURLToURL oldkey newbackend _af _

Backend/Variety.hs

@@ -29,7 +29,7 @@ regularBackendList = Backend.Hash.backends
 
 {- The default hashing backend. -}
 defaultHashBackend :: Backend
-defaultHashBackend = Prelude.head regularBackendList
+defaultHashBackend = fromMaybe (error "internal") $ headMaybe regularBackendList
 
 makeVarietyMap :: [Backend] -> M.Map KeyVariety Backend
 makeVarietyMap l = M.fromList $ zip (map backendVariety l) l