bug#9642: move-overlay creates an empty overlay with the evaporate property

[Stefan Monnier]

>> returns an empty overlay that has the evaporate property.
>> But this is not supposed to happen: when an overlay with that
>> property becomes empty, it's supposed to be deleted.

> OTOH if the user goes through the trouble to do the above, he probably
> doesn't want this overlay to disappear right away.  So I'm not sure we
> want to second guess the user here.

We can discuss it now, but in any case I don't think we should change
this behavior for 24.1.


        Stefan