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.

Git/Command.hs

@@ -24,10 +24,10 @@ gitCommandLine params r@(Repo { location = l@(Local { } ) }) =
   where
 	setdir
 		| gitEnvOverridesGitDir r = []
-		| otherwise = [Param $ "--git-dir=" ++ gitdir l]
+		| otherwise = [Param $ "--git-dir=" ++ fromRawFilePath (gitdir l)]
 	settree = case worktree l of
 		Nothing -> []
-		Just t -> [Param $ "--work-tree=" ++ t]
+		Just t -> [Param $ "--work-tree=" ++ fromRawFilePath t]
 gitCommandLine _ repo = assertLocal repo $ error "internal"
 
 {- Runs git in the specified repo. -}