[PATCH 5 of 5 V2] revsets: add new topographical sort

[Yuya Nishihara]

On Tue, 24 May 2016 11:11:20 -0400, Augie Fackler wrote:
> I think we should consider breaking some of the revset predicates into
> separate files (3k lines is past the point where we probably should
> have considered some better organization IMO). Maybe the sorts could
> live in a revsetsorts.py or something? Just something to think about
> long-term, not something that's suitable for this series.

Perhaps splitting smartset classes into core module would be good start.
They consumes ~1k lines.