Make "path already scanned" message deterministic (#820)

When a symlink loop is detected, the following message is printed:

> Path X has already been scanned. Cannot scan it again through symlink Y

However, the actual symlink that is displayed depends on the order in
with `readdir` outputs the contents (which depends on FS internals).

This sorts the directory list so that the message is deterministic.

It can be tested by adding for example a `List.rev` call after
`Path.readdir`: the affected version will return a different version in
the `github764` test.

Signed-off-by: Etienne Millon <etienne@cryptosense.com>

src/file_tree.ml

@@ -199,6 +199,11 @@ let load ?(extra_ignored_subtrees=Path.Set.empty) path =
             Left fn)
       in
       let files = String.Set.of_list files in
+      let sub_dirs =
+        List.sort
+          ~compare:(fun (a, _, _) (b, _, _) -> String.compare a b)
+          sub_dirs
+      in
       let project =
         match Dune_project.load ~dir:path ~files with
         | Some _ as x -> x