convert TopFilePath to use RawFilePath

Adds a dependency on filepath-bytestring, an as yet unreleased fork of
filepath that operates on RawFilePath.

Git.Repo also changed to use RawFilePath for the path to the repo.

This does eliminate some RawFilePath -> FilePath -> RawFilePath
conversions. And filepath-bytestring's </> is probably faster.
But I don't expect a major performance improvement from this.
This is mostly groundwork for making Annex.Location use RawFilePath,
which will allow for a conversion-free pipleline.

Backend/WORM.hs

@@ -38,7 +38,8 @@ keyValue source _ = do
 	let f = contentLocation source
 	stat <- liftIO $ getFileStatus f
 	sz <- liftIO $ getFileSize' f stat
-	relf <- getTopFilePath <$> inRepo (toTopFilePath $ keyFilename source)
+	relf <- fromRawFilePath . getTopFilePath
+		<$> inRepo (toTopFilePath $ toRawFilePath $ keyFilename source)
 	return $ Just $ mkKey $ \k -> k
 		{ keyName = genKeyName relf
 		, keyVariety = WORMKey