remove Read instance for Ref

Removed instance, got it all to build using fromRef. (With a few things
that really need to show something using a ref for debugging stubbed out.)

Then added back Read instance, and made Logs.View use it for serialization.
This changes the view log format.

Git/Objects.hs

@@ -32,4 +32,4 @@ listLooseObjectShas r = catchDefaultIO [] $
 looseObjectFile :: Repo -> Sha -> FilePath
 looseObjectFile r sha = objectsDir r </> prefix </> rest
   where
-	(prefix, rest) = splitAt 2 (show sha)
+	(prefix, rest) = splitAt 2 (fromRef sha)