Add one more important step to GenGC algo.
Add one more important step to GenGC algo.

Now we should handle C-stack nursery objects in a sane way.

gagern/parrot,FROGGS/parrot,gitster/parrot,youprofit/parrot,gagern/parrot,tewk/parrot-select,tewk/parrot-select,tewk/parrot-select,gitster/parrot,gitster/parrot,youprofit/parrot,gagern/parrot,tkob/parrot,FROGGS/parrot,tkob/parrot,tkob/parrot,FROGGS/parrot,gagern/parrot,parrot/parrot,tkob/parrot,parrot/parrot,tewk/parrot-select,tkob/parrot,youprofit/parrot,gagern/parrot,youprofit/parrot,gitster/parrot,youprofit/parrot,tewk/parrot-select,FROGGS/parrot,parrot/parrot,gagern/parrot,gagern/parrot,FROGGS/parrot,youprofit/parrot,tewk/parrot-select,tkob/parrot,tkob/parrot,gitster/parrot,youprofit/parrot,FROGGS/parrot,tewk/parrot-select,FROGGS/parrot,gitster/parrot,tkob/parrot,parrot/parrot,gitster/parrot,youprofit/parrot,parrot/parrot,FROGGS/parrot