Infinite loop when reverting back to last saved state

Under certain conditions it is possible that rolling back changes when not saving a model (e.g., when going back to the feature model) an infinite loop can occur.

This happens in CORECommandStack.goToLastSave(), because isSaveNeeded() always returns true.

The reason this happens is that:

saveIndex = -2
top = -1 or higher

I found in the source code of BasicCommandStack the following snippet:

// This is kind of tricky.
// If the saveIndex was in the redo part of the command list which has now been wiped out,
// then we can never reach a point where a save is not necessary, not even if we undo all the way back to the beginning.
//
if (saveIndex >= top)
{
  // This forces isSaveNeded to always be true.
  //
  saveIndex = -2;
}

I.e., if the user undoes some commands and then makes changes, the saveIndex will be set to -2, because the saved state can never be reached. This means that we need to be able to handle this special case somehow.

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