Add support to incrementally updating "What's in" report

Add support to incrementally updating "What's in" report

If the result of previous run is in the filesystem but not committed yet,
use that as a template.  This mode needs to behave slightly differently in
that the topics that appeared in the template file's "New Topics" section
need to be kept, and "New Topics" section should not get renamed to "Old
New Topics".

The sections need to be kept together after this change, and also the
handling of the Graduated to "master" psuedo section should also be
straightened out, but that can come later.