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/Merger.hs

@@ -26,7 +26,7 @@ import qualified Command.Sync
 mergeThread :: NamedThread
 mergeThread = namedThread "Merger" $ do
 	g <- liftAnnex gitRepo
-	let dir = Git.localGitDir g </> "refs"
+	let dir = fromRawFilePath (Git.localGitDir g) </> "refs"
 	liftIO $ createDirectoryIfMissing True dir
 	let hook a = Just <$> asIO2 (runHandler a)
 	changehook <- hook onChange