discovery: move code to create outgoing from roots and heads changegroup.changegroupsubset() contained somewhat low-level code for constructing an "outgoing" instance from a list of roots and heads nodes. It feels like discovery.py is a more appropriate location for this code. This code can definitely be optimized, as outgoing.missing will recompute the set of changesets we've already discovered from cl.between(). But code shouldn't be refactored during a move, so I've simply inserted a TODO calling attention to that.

Ancestors of 5
4 2 0 
Ancestors of 6 and 5
3 4 2 1 0 
Ancestors of 5 and 4
4 2 0 
Ancestors of 7, stop at 6
6 
Ancestors of 7, including revs
7 6 5 3 4 2 1 0 
Ancestors of 7, 5 and 3, including revs
7 5 3 6 4 2 1 0 

Descendants of 5
7 8 
Descendants of 5 and 3
6 7 8 
Descendants of 5 and 4
5 7 8