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.

Command/ResolveMerge.hs

@@ -24,7 +24,7 @@ seek = withNothing (commandAction start)
 start :: CommandStart
 start = starting "resolvemerge" (ActionItemOther Nothing) $ do
 	us <- fromMaybe nobranch <$> inRepo Git.Branch.current
-	d <- fromRepo Git.localGitDir
+	d <- fromRawFilePath <$> fromRepo Git.localGitDir
 	let merge_head = d </> "MERGE_HEAD"
 	them <- fromMaybe (error nomergehead) . extractSha
 		<$> liftIO (readFile merge_head)