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/WebApp/DashBoard.hs

@@ -118,7 +118,8 @@ getFileBrowserR = whenM openFileBrowser redirectBack
  - blocking the response to the browser on it. -}
 openFileBrowser :: Handler Bool
 openFileBrowser = do
-	path <- liftIO . absPath =<< liftAnnex (fromRepo Git.repoPath)
+	path <- liftIO . absPath . fromRawFilePath
+		=<< liftAnnex (fromRepo Git.repoPath)
 #ifdef darwin_HOST_OS
 	let cmd = "open"
 	let p = proc cmd [path]