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

[Stefan Kangas <stefan@marxist.se>]

Stefan Kangas <stefan@marxist.se> writes:

>  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.

I started looking into a solution with {before,after}-revert-hook, but
that just seems duplicate what we would get from just using a minor
mode.  So I'm starting to think that `elide-head' and `elide-head-show'
should be obsoleted in favor of a new local minor mode
`elide-head-mode'.

That would also save me from having to add a third command
`elide-head-toggle' to toggle the hiding on or off.