From the announce:
"Mutt 1.5.18 contains 6 months of bug fixes, documentation improvements and
performance enhancements since the release of 1.5.17, but I believe only
one new feature ($time_inc, for controlling the rate at which status updates
are displayed)."
This release is largely a bug-fix release, but it does contain a couple of
small new features (next-unread-mailbox, $message_cache_clean).
The "change_folder_next" patches are gone, since a similar feature was added,
and it is now possible to use
bind index , next-unread-mailbox
to use , to cycle mailboxes with new mail
