changegroup: extract method that sorts nodes to send
The current implementation of narrowhg needs to influence the order in
which nodes are sent to the client. adgar@ and I think this is
fixable, but it's going to require pretty substantial time investment,
so in the interim we'd like to extract this method.
I think it makes the group() code a little more obvious, as it took us
a couple of tries to isolate the exact behavior we were observing.