"git pack-objects" learned another algorithm to compute the set of
objects to send, that trades the resulting packfile off to save
traversal cost to favor small pushes.

* ds/push-sparse-tree-walk:
  pack-objects: create GIT_TEST_PACK_SPARSE
  pack-objects: create pack.useSparse setting
  revision: implement sparse algorithm
  list-objects: consume sparse tree walk
  revision: add mark_tree_uninteresting_sparse