better directory handling

Rename Locations functions for better consitency, and make their values
more consistent too.

Used </> rather than manually building paths. There are still more places
that manually do so, but are tricky, due to the behavior of </> when
the second FilePath is absolute. So I only changed places where
it obviously was relative.

Command/Unused.hs

@@ -43,7 +43,7 @@ checkUnused = do
 	unused <- unusedKeys
 	let list = number 1 unused
 	g <- Annex.gitRepo
-	liftIO $ writeFile (annexUnusedLog g) $ unlines $ 
+	liftIO $ writeFile (gitAnnexUnusedLog g) $ unlines $ 
 		map (\(n, k) -> show n ++ " " ++ show k) list
 	if null unused
 		then return True