"git pack-objects" in a repository with many packfiles used to
spend a lot of time looking for/at objects in them; the accesses to
the packfiles are now optimized by checking the most-recently-used
packfile first.

* jk/pack-objects-optim-mru:
  pack-objects: use mru list when iterating over packs
  pack-objects: break delta cycles before delta-search phase
  sha1_file: make packed_object_info public
  provide an initializer for "struct object_info"