Fix logic inside depgraph._serialize_tasks() to avoid the circular runtime

Fix logic inside depgraph._serialize_tasks() to avoid the circular runtime
deps path in some cases when it's not appropriate. This solves a case that
was reported, in which the perl was merged before libperl due do perl and
lots of it's deps being selected all at once. In this case, so many packages
were selected at once that the cmp_circular_bias() sort did not order them
very well (though it normally works fine with a smaller number of packages).
Thanks to Daniel Robbins for reporting this issue and helping me reproduce
it. (trunk r12568)

svn path=/main/branches/2.1.6/; revision=12849