bug#30823: 25.3; modification-hooks of overlays are not run in some cases

[Eli Zaretskii]

> From: Richard Stallman <rms@gnu.org>
> Cc: eliz@gnu.org, victorhge@gmail.com, npostavs@gmail.com,
>       30823@debbugs.gnu.org
> Date: Sun, 19 Aug 2018 23:02:54 -0400
> 
>   >   I was asking
>   > > whether we want to support code which does this, because maybe it is
>   > > unreasonable to delete an overlay from within its modification hook.
> 
> I think I see natural occasions to want to do just that, so I think we had
> better support it.

I have no doubt that it's be nice to have.  However, there are
practical difficulties with allowing that, and in particular a simple
enough device I added to try to support it doesn't work in a slightly
more complicated case.  When this stuff fails, it is usually goes up
in smoke, and debugging that is not easy.  So if someone has better
ideas, patches are welcome.