concat Echo args with space rather than blank

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/action.ml

@@ -688,7 +688,7 @@ let rec exec t ~ectx ~dir ~env ~stdout_to ~stderr_to =
     exec t ~ectx ~dir ~stdout_to ~stderr_to
       ~env:(Env.add env ~var ~value)
   | Redirect (Stdout, fn, Echo s) ->
-    Io.write_file fn (String.concat s ~sep:"");
+    Io.write_file fn (String.concat s ~sep:" ");
     Fiber.return ()
   | Redirect (outputs, fn, Run (Ok prog, args)) ->
     let out = Process.File fn in
@@ -705,7 +705,7 @@ let rec exec t ~ectx ~dir ~env ~stdout_to ~stderr_to =
     redirect ~ectx ~dir outputs Config.dev_null t ~env ~stdout_to ~stderr_to
   | Progn l ->
     exec_list l ~ectx ~dir ~env ~stdout_to ~stderr_to
-  | Echo strs -> exec_echo stdout_to (String.concat strs ~sep:"")
+  | Echo strs -> exec_echo stdout_to (String.concat strs ~sep:" ")
   | Cat fn ->
     Io.with_file_in fn ~f:(fun ic ->
       let oc =

src/value.ml

@@ -23,7 +23,7 @@ let paths = List.map ~f:(fun x -> Path x)
 
 let concat ts ~dir =
   List.map ~f:(to_string ~dir) ts
-  |> String.concat ~sep:""
+  |> String.concat ~sep:" "
 
 let paths_only =
   List.filter_map ~f:(function