Fix missing directory filename separator

lib/file_store.ml

@@ -44,7 +44,7 @@ let iter_valid_text dir pred fn p =
 let fold_valid_text dir pred fn acc p =
   if not (text_filetype dir p) then acc else
     match to_text dir p with
-    Error x -> prerr_endline x; acc | Ok t -> if pred t then fn acc (t, dir^p) else acc
+    Error x -> prerr_endline x; acc | Ok t -> if pred t then fn acc (t, dir//p) else acc
 
 let iter ?(predicate=fun _ -> true) ?order fn {store;_} =
   match order with