use filepath-bytestring for annex object manipulations

git-annex find is now RawFilePath end to end, no string conversions.
So is git-annex get when it does not need to get anything.
So this is a major milestone on optimisation.

Benchmarks indicate around 30% speedup in both commands.

Probably many other performance improvements. All or nearly all places
where a file is statted use RawFilePath now.

Command/DiffDriver.hs

@@ -90,7 +90,8 @@ fixupReq req@(Req {}) =
 			v <- getAnnexLinkTarget' (toRawFilePath (getfile r)) False
 			case parseLinkTargetOrPointer =<< v of
 				Nothing -> return r
-				Just k -> withObjectLoc k (pure . setfile r)
+				Just k -> withObjectLoc k $
+					pure . setfile r . fromRawFilePath
 		_ -> return r
 
 externalDiffer :: String -> [String] -> Differ