fix filename encoding for git cat-file

The filename sent to git cat-file needs to be sent on a File encoded handle.

Also set the read handle to use the File encoding, so that any error
message mentioning the filename is received properly.

The actual file content is read using Data.ByteString.Char8, which
will ignore the read handle's encoding, so this won't change that.
(Whether that is entirely correct remains to be seen.)

Git/CatFile.hs

@@ -43,8 +43,11 @@ catFile h branch file = catObject h $ Ref $ show branch ++ ":" ++ file
 catObject :: CatFileHandle -> Ref -> IO L.ByteString
 catObject h object = CoProcess.query h send receive
 	where
-		send to = hPutStrLn to $ show object
+		send to = do
+			fileEncoding to
+			hPutStrLn to $ show object
 		receive from = do
+			fileEncoding from
 			header <- hGetLine from
 			case words header of
 				[sha, objtype, size]
@@ -56,7 +59,7 @@ catObject h object = CoProcess.query h send receive
 					| otherwise -> dne
 				_
 					| header == show object ++ " missing" -> dne
-					| otherwise -> error $ "unknown response from git cat-file " ++ header
+					| otherwise -> error $ "unknown response from git cat-file " ++ show (header, object)
 		readcontent bytes from = do
 			content <- S.hGet from bytes
 			c <- hGetChar from