extmsg: don't bother partial matching with <16 chars
It's not worth it, and attempts to wildcard off
single-character Message-IDs(*) causes Xapian to error
out in unpredictable ways:
something terrible happened at /usr/lib/x86_64-linux-gnu/perl5/5.24/Search/Xapian/Enquire.pm line 54.
...propagated at lib/PublicInbox/Search.pm line 209.
So don't bother.
(*) because people blindly hit 'y' or 'n' when git-send-email
prompted them for In-Reply-To.