This appears to significantly improve header caching behavior
with mutt. With the current public-inbox.org/git mirror(*),
mutt will only re-FETCH the last ~300 or so messages in the
final "inbox.comp.version-control.git.7" mailbox, instead of
~49,000 messages every time.
It's not perfect, but a 500ms query is better than a >10s query
and mutt itself spends as much time loading its header cache.
(*) there are many gaps in NNTP article numbers (UIDs) due to
spam removal from public-inbox-learn.