Use longer include lists in GC descriptors

[Evan Kirshenbaum]

[imported from HPE issue 137]

Currently, when faced with the choice of using an include list or an exclude list in a descriptor, we choose the shorter one. When exactly half of the fields are references, we go with an include list, since they are cheaper to process (I think--need to check if this is always true). When either is possible, we may want to go with the include list instead, even if this results in a longer list. It's certainly cheaper to exclude zero fields, and it's probably cheaper to exclude one. But if the choice is to exclude three of ten or include seven, we're probably better off with the include. Maybe.