Merge branch 'jk/packfile-reuse-cleanup' into jch

Merge branch 'jk/packfile-reuse-cleanup' into jch

The way "git pack-objects" reuses objects stored in existing pack
to generate its result has been improved.

* jk/packfile-reuse-cleanup:
  pack-objects: improve partial packfile reuse
  builtin/pack-objects: introduce obj_is_packed()
  pack-objects: introduce pack.allowPackReuse
  csum-file: introduce hashfile_total()
  pack-bitmap: introduce bitmap_walk_contains()
  pack-bitmap: don't rely on bitmap_git->reuse_objects
  ewah/bitmap: introduce bitmap_word_alloc()
  packfile: expose get_delta_base()
  builtin/pack-objects: report reused packfile objects