convert git-annex branch access to ByteStrings and Builders

Most of the individual logs are not converted yet, only presense logs
have an efficient ByteString Builder implemented so far. The rest
convert to and from String.

Upgrade/V2.hs

@@ -83,7 +83,7 @@ inject source dest = do
 	old <- fromRepo olddir
 	new <- liftIO (readFile $ old </> source)
 	Annex.Branch.change dest $ \prev -> 
-		unlines $ nub $ lines prev ++ lines new
+		encodeBL $ unlines $ nub $ lines (decodeBL prev) ++ lines new
 
 logFiles :: FilePath -> Annex [FilePath]
 logFiles dir = return . filter (".log" `isSuffixOf`)