[notmuch] Backport of Xapian term update optimisation

[Olly Betts]

I've backported the term update optimisation patches
<http://trac.xapian.org/ticket/250> to Xapian's 1.0 branch, and you can
find snapshot tarballs including these changes here:

http://oligarchy.co.uk/xapian/branches/1.0/

Xapian's testsuite passes (including the additional test coverage which I
also backported), and I looked over each change carefully, but I would be
interested to see some real world testing, particularly in the situation
which these changes are intended to improve (i.e. speed of tagging in
notmuch).

So if you're so inclined, try it and report how you got on (on this list
is fine).

Cheers,
    Olly