Clear console before running command in exec -w (ocaml#10983)

This fixes an issue where the build progress stays on the console after
the build completes, and the first line of the output of the command
being exec'd is printed on the same line as the build progress.

Signed-off-by: Stephen Sherratt <stephen@sherra.tt>

bin/exec.ml

@@ -94,10 +94,17 @@ module Command_to_exec = struct
 
   (* Run the command, first (re)building the program which the command is
      invoking *)
-  let build_and_run_in_child_process ~root { get_env_and_build_if_necessary; prog; args } =
+  let build_and_run_in_child_process
+    ~root
+    ~config
+    { get_env_and_build_if_necessary; prog; args }
+    =
     get_env_and_build_if_necessary prog
     |> Fiber.map
-         ~f:(Result.map ~f:(fun { path; env } -> spawn_process ~root ~args ~env path))
+         ~f:
+           (Result.map ~f:(fun { path; env } ->
+              Scheduler.maybe_clear_screen ~details_hum:[] config;
+              spawn_process ~root ~args ~env path))
   ;;
 end
 
@@ -144,18 +151,18 @@ module Watch = struct
 
   (* Kills the currently running process, then runs the given command after
      (re)building the program which it will invoke *)
-  let run ~root state ~command_to_exec =
+  let run ~root ~config state ~command_to_exec =
     let open Fiber.O in
     let* () = Fiber.return () in
     let* () = kill_currently_running_process state in
     let* command_to_exec = command_to_exec () in
-    Command_to_exec.build_and_run_in_child_process ~root command_to_exec
+    Command_to_exec.build_and_run_in_child_process ~root ~config command_to_exec
     >>| Result.map ~f:(fun pid -> state.currently_running_pid := Some pid)
   ;;
 
-  let loop ~root ~command_to_exec =
+  let loop ~root ~config ~command_to_exec =
     let state = init_state () in
-    Scheduler.Run.poll (run ~root state ~command_to_exec)
+    Scheduler.Run.poll (run ~root ~config state ~command_to_exec)
   ;;
 end
 
@@ -328,7 +335,7 @@ module Exec_context = struct
       ; args
       }
     in
-    Watch.loop ~root:(Common.root common) ~command_to_exec
+    Watch.loop ~root:(Common.root common) ~config ~command_to_exec
   ;;
 end