Reduce volume of Travis build output

Hundreds of lines of build output passed through Travis JS parsing bring
my browser (at least) to its knees - little of it is interesting or even
relevant, keeping the last 40 lines of stdout for most make targets is
plenty (we keep all of stderr).
Avoid misleading messages about change id 0

Fixes bug 2618
Handle exceptions when there are no changes

If no edits recorded, catch DoesNotExist; treat as latest_change_id = 0
Make 0/None 'modified-since' logic explicit