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

* jc/maint-1.6.0-keep-pack: 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