[BUGFIX beta] reduceComputed detect retain:n better.

Fix #4620.

reduceComputed tries to skip walking all observer contexts when an item
property changes if the current tracked array state is [retain:n].  However,
this check occurs while applying the operations.  If [delete:m retain:(n-m)]
is applied, when the retain operation is visisted it will have been updated to
retain:n.

This commit updates the check so that retain:n is only checked for the *first* 
operation.