Pierre-Yves David <pierre-yves.david@fb.com> [Tue, 23 Jun 2015 00:02:23 -0700] rev 1421
rework refactor _aspiringchildren by introducing _possibledestination
This allows us to reuse some of the logic for evolve from _aspiringchildren
for the new implementation of evolve --all.
The logic is also better as some previously selected changesets may not actually
evolve on the target, and some changesets that does not would not.