Limit summary index to entries with >0 changes

In my sample data, runs with changes are only about 10% of the total, so
limiting it to only indexing non-zero changes is a considerable
reduction of the load of summary indexing. (The downside, of course, is
that you do not have the confirmation that the runs where otherwise
happening, but the load of the summary indexing was getting to be a
problem so this was well worth it for me.)