Add a progress reporter to recursive progress discovery

[juergenhoetzel]

Discovering a deep recursive directory structure can take a while to finish.
Inform user about the progress so the user can estimate remaining time
and clearly see that Emacs is busy working, not hung.

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[bbatsov]

Nice improvement. Thanks!

[juergenhoetzel]

Nice improvement. Thanks!

You are welcome: @bbatsov the fastet merger at @github 🤣