optimise catfile interface with ByteString and Attoparsec

Around 3% total speedup.

Profiling git annex find --not --in web, it's now bytestring end-to-end,
and there is only a little added overhead in eg accessing the Annex
state MVar (3%). The rest of the runtime is spent reading symlinks, and
in attoparsec.

This feels like the end of the optimisation road, without a major change
like caching information for faster queries.

Git/Types.hs

@@ -101,6 +101,7 @@ newtype RefDate = RefDate String
 
 {- Types of objects that can be stored in git. -}
 data ObjectType = BlobObject | CommitObject | TreeObject
+	deriving (Show)
 
 readObjectType :: S.ByteString -> Maybe ObjectType
 readObjectType "blob" = Just BlobObject