Hey all,

It occurred to me that if you're not using the Gnus registry's
split-with-parent functionality, there really isn't any need to let it
bloat up with thousands of entries. If all you're doing with it is
setting registry marks on messages (or using Gnorb's Gnus<->Org
tracking, or message tagging), the only entries we ever need are those
where we've explicitly added "precious" information to a message.

The attached patch does that, I think (modulo stuff I haven't thought
of). I called it `gnus-registry-precious-only-p' because I originally
thought it would do some filtering on Summary exit or something like
that, but it turned out to be simpler than that: all the commands I'm
aware of that set precious information on an entry do so with a
"get-or-create" lookup. Meaning that setting the information is enough
to create the entry, meaning all we have to do is not create entries
automatically.

So this option is a tiny bit of a misnomer, but I still think it's
appropriate because pruning will avoid precious entries, and the end
effect is that the only entries you're left with are those with precious
information. If you delete that information later, the entry will be
eligible for pruning. Most people will have a relatively tiny number of
precious entries, so if you're not using the registry for splitting this
could save you a lot of space and save time.

WDYT?

[Gnorb-related addendum: I've never used registry split-with-parent,
because it seemed like the potential for chaos was high. But if we only
keep a very controlled number of entries in the registry, this could
provide an alternate mechanism for creating groups that hold messages
related to an Org heading. Gnorb can do that now with ephemeral groups,
but these would be real mail groups: track the first message, move it to
a group "about" an Org heading, and subsequent replies to that message
will get moved as well. Hmm, could messages be copied instead of
moved...?]

Eric