git-rev-list: remove the DUPCHECK logic, use SEEN instead

git-rev-list: remove the DUPCHECK logic, use SEEN instead

That's what we should have done in the first place, since it not only
avoids another unnecessary flag, it also protects the commits from
showing up as duplicates later when they show up as parents of another
commit (in the pop_most_recent_commit() path).

This will hopefully also fix --topo-sort.