Due to an off-by-one error, we were unable to read mailboxes
with only a single message of UID:1. Without this fix, the
message with UID:1 could only be read after UID:2 was created;
so there's no permanent data loss as long as a new message
showed up.
This affects all releases of public-inbox-watch with IMAP
support, though it probably went unnoticed because single
message inboxes are rare.