Merge branch 'jc/maint-1.6.0-keep-pack' into maint-1.6.1

Merge branch 'jc/maint-1.6.0-keep-pack' into maint-1.6.1

* jc/maint-1.6.0-keep-pack:
  pack-objects: don't loosen objects available in alternate or kept packs
  t7700: demonstrate repack flaw which may loosen objects unnecessarily
  Remove --kept-pack-only option and associated infrastructure
  pack-objects: only repack or loosen objects residing in "local" packs
  git-repack.sh: don't use --kept-pack-only option to pack-objects
  t7700-repack: add two new tests demonstrating repacking flaws
  is_kept_pack(): final clean-up
  Simplify is_kept_pack()
  Consolidate ignore_packed logic more
  has_sha1_kept_pack(): take "struct rev_info"
  has_sha1_pack(): refactor "pretend these packs do not exist" interface
  git-repack: resist stray environment variable