Use the reference depth approach also for changed cell regions.

Otherwise, we'll get unnecessary recalculation events in some cases. In 1.5
some cells were recalculated twice, if one of its references were changed.

Example:
A2=A1+A3
A3=A1
Changing A1 has recalced A2 first, which triggered a recalc of A3, which in turn recalced A2 again.

TODO: Catch circular dependencies.