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/MountWatcher.hs

@@ -159,7 +159,7 @@ handleMount urlrenderer dir = do
  -}
 remotesUnder :: FilePath -> Assistant [Remote]
 remotesUnder dir = do
-	repotop <- liftAnnex $ fromRepo Git.repoPath
+	repotop <- liftAnnex $ fromRawFilePath <$> fromRepo Git.repoPath
 	rs <- liftAnnex remoteList
 	pairs <- liftAnnex $ mapM (checkremote repotop) rs
 	let (waschanged, rs') = unzip pairs