Restoring vtkRedistributeDataSetFilter behavior

!8812 (merged) broke a functionality of vtkRedistributeDataSetFilter. When setting NumberOfPartitions to some number larger than the number of input partitions, the filter internally generates a partitioned data set as a temporary state. When setting PreservePartitionsInOutput to false, the output becomes a vtkUnstructuredGrid and we need to merge the partitions into one partition. Instead of merging, the filter was just saving the first partition.

The trick to have both this behavior working and not having duplicate points in the output is to set MergingPointsOn in the appender that merges the partitions into the output.

Fixes #18632 (closed)

Backport: release