We were accidentally adding "\n" to terms (which Xapian happily
accepts), causing incompatibilities when enabling parallel
sharding in some invocations of -extindex but not others.
This is an extindex incompatibility and starting a new extindex
will be required to take advantage of in-development features,
so it's not urgent to start another one, either.
(other incompatible things may happen before a 1.7 release)