While looking into 2550676 I noticed some of the Xapian API usage in
roundup could be improved.  Being able to pass a term to
"replace_document" is an old feature (added in Xapian 0.8.2, roundup
requires Xapian > 1.0).  This change should speed up indexing a little.

Also, add_term() is better than add_posting() since it is pointless to
be storing positional information for these terms.  This change will
reduce database size a little.

The patch is untested I'm afraid - I will try to find time to test it,
but thought it worth filing this so the patch doesn't get lost, and so
anyone enthusiastic can look at it if they want.