discovery: move code to create outgoing from roots and heads...
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.
