bug#52980: elide-head and revert-buffer interaction stops elide-head-show from working

[Stefan Kangas]

On current master:

 0. emacs -Q lisp/help.el
        ;; Any file where the header would normally be elided
 1. M-x elide-head RET
 2. M-x revert-buffer RET
 3. M-: elide-head-overlay RET  => nil
        ;; Note that the overlay is still in the buffer though.
 4. M-x elide-head RET
        ;; A second overlay is added.
 5. M-x elide-head-show RET

Now I see no license, as only the second overlay is removed.

Note that the above sequence of commands should be equivalent to what
happens when you revert the buffer if you have added `elide-head' to a
major mode hook, as in:

  (add-hook 'prog-mode-hook 'elide-head)