This fixes CVE_2020-24386, CVE-2020-25725 and a bunch of regular bugs
[1].
* CVE-2020-24386: Specially crafted command can cause IMAP hibernate to
allow logged in user to access other people's emails and filesystem
information.
* CVE-2020-25275: Mail delivery / parsing crashed when the 10 000th MIME part was
message/rfc822 (or if parent was multipart/digest). This happened
due to earlier MIME parsing changes for CVE-2020-12100.
[1] https://raw.githubusercontent.com/dovecot/core/2.3.13/NEWS
58c7d3ff17