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.

Assistant/Threads/WebApp.hs

@@ -100,7 +100,7 @@ webAppThread assistantdata urlrenderer noannex cannotrun postfirstrun listenhost
 	getreldir
 		| noannex = return Nothing
 		| otherwise = Just <$>
-			(relHome =<< absPath
+			(relHome =<< absPath . fromRawFilePath
 				=<< getAnnex' (fromRepo repoPath))
 	go tlssettings addr webapp htmlshim urlfile = do
 		let url = myUrl tlssettings webapp addr