[notmuch] Backport of Xapian term update optimisation

[Olly Betts]

Thanks for all the testing feedback folks.

I've made a new Xapian release (1.0.18) and packaged it for Debian.  If you
are using unstable, and not on mips or hurd, it's now built:

https://buildd.debian.org/status/package.php?p=xapian-core

I haven't updated the xapian.org download page, etc yet - I wanted to get the
Debian package uploaded promptly so there's more chance it'll get into the
next Ubuntu release - but you can find tarballs in the usual place:

http://oligarchy.co.uk/xapian/1.0.18/

Cheers,
    Olly