changegroup: reintroduce some comments that have gotten lost over the years I got concerned about the correctness of the pruning logic, but I was misreading it. I didn't figure that out until I walked all the way back to 0252abaafb8a from 20111, where I was finally able to see (in the deleted side of the change!) a complete explanation from b6d9ea0bc107 in 2005. Differential Revision: https://phab.mercurial-scm.org/D4686