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/Migrate.hs

@@ -50,7 +50,7 @@ perform file oldkey newbackend = do
 	-- (the file can't be stored as usual, because it's already a symlink).
 	-- The old backend's key is not dropped from it, because there may
 	-- be other files still pointing at that key.
-	let src = annexLocation g oldkey
+	let src = gitAnnexLocation g oldkey
 	stored <- Backend.storeFileKey src $ Just newbackend
 	case stored of
 		Nothing -> return Nothing