msgmap: mid_insert: use plain "INSERT" to detect duplicates
"INSERT OR IGNORE" still bumps the auto-increment counter in
SQLite, which causes gaps to appear in NNTP article numbering.
This bug appeared in v2 repos where V2Writable may call ->add
repeatedly on the same message. This bug is apparent with
public-inbox-watch and work-in-progress IMAP watchers which may
rescan and (attempt to) reinsert the same message on mailbox
changes.
Most uses of public-inbox-mda were not affected, unless the
same message is actually delivered multiple times to the mda.
v1 is not affected, either, since deduplication is only based
on Message-ID and msgmap never sees the duplicate.
Reported-by: "Eric W. Biederman" <ebiederm@xmission.com>