avoid exponential resource consumption

the cumulative mode tries hard to never lose already assigned variable
values. the naive implementation turned assignments into additions,
which made the trivial construct "FOO = $$FOO" double the size of the
array each time it would be executed.
so instead let the assignments be assignments, and then re-add the
values that would be dropped.

Reviewed-by: dt
Task-number: QTCREATORBUG-1595