improve type signatures with a Ref newtype

In git, a Ref can be a Sha, or a Branch, or a Tag. I added type aliases for
those. Note that this does not prevent mixing up of eg, refs and branches
at the type level. Since git really doesn't care, except rare cases like
git update-ref, or git tag -d, that seems ok for now.

There's also a tree-ish, but let's just use Ref for it. A given Sha or Ref
may or may not be a tree-ish, depending on the object type, so there seems
no point in trying to represent it at the type level.

Git/CatFile.hs

@@ -37,14 +37,14 @@ catFileStop (pid, from, to) = do
 	forceSuccess pid
 
 {- Reads a file from a specified branch. -}
-catFile :: CatFileHandle -> String -> FilePath -> IO L.ByteString
-catFile h branch file = catObject h (branch ++ ":" ++ file)
+catFile :: CatFileHandle -> Branch -> FilePath -> IO L.ByteString
+catFile h branch file = catObject h $ Ref $ show branch ++ ":" ++ file
 
 {- Uses a running git cat-file read the content of an object.
  - Objects that do not exist will have "" returned. -}
-catObject :: CatFileHandle -> String -> IO L.ByteString
+catObject :: CatFileHandle -> Ref -> IO L.ByteString
 catObject (_, from, to) object = do
-	hPutStrLn to object
+	hPutStrLn to $ show object
 	hFlush to
 	header <- hGetLine from
 	case words header of
@@ -53,7 +53,7 @@ catObject (_, from, to) object = do
 			  validobjtype objtype -> handle size
 			| otherwise -> empty
 		_
-			| header == object ++ " missing" -> empty
+			| header == show object ++ " missing" -> empty
 			| otherwise -> error $ "unknown response from git cat-file " ++ header
 	where
 		handle size = case reads size of