SI-8910 BitSet sometimes uses exponential memory.

Because of an off-by-one error in scala.collection.mutable.BitSet, where a function (ensureCapacity) is passed a list length instead of an index, when ^=, &=, |=, or &~= are passed BitSets with the same internal capacity as the set the method is being invoked on, the size of the first BitSet is needlessly doubled.

This patch simply changes the ensureCapacity calls to pass the last index of the list, instead of the raw length.