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/Index.hs

@@ -49,7 +49,7 @@ override index _r = do
 
 {- The normal index file. Does not check GIT_INDEX_FILE. -}
 indexFile :: Repo -> FilePath
-indexFile r = localGitDir r </> "index"
+indexFile r = fromRawFilePath (localGitDir r) </> "index"
 
 {- The index file git will currently use, checking GIT_INDEX_FILE. -}
 currentIndexFile :: Repo -> IO FilePath