Fix displayed number of archived tasks when archiving subtasks

[lebigot]

When a task has subtasks and is archived (with the "nested" command, at least), the number of archived tasks reported is only the number of top-level tasks.

I was surprised by this and thought that some of the tasks that had disappeared had not been archived, as the number was too low. I would suggest that the total number of tasks and subtasks that were archived be displayed instead (or, possibly, the number of tasks and subtasks, but this doesn't scale with sub-sub-tasks, etc.).