bug#30943: save-hist creates massive cache file

[Glenn Morris]

Eli Zaretskii wrote:

> Save-hist doesn't truncate any variables except the ones it knows
> about, because it cannot be sure that truncating some list might leave
> the variable with an invalid value.

savehist doesn't truncate anything AFAICS.
It just assumes variables are trimmed by add-to-history etc in the
course of normal operation. So if you add a non-history variable to
savehist (or set history-length to t), it will eventually blow up.