Merge branch 'jk/maint-pack-objects-compete-with-delete'

* jk/maint-pack-objects-compete-with-delete:
  downgrade "packfile cannot be accessed" errors to warnings
  pack-objects: protect against disappearing packs

diff --cc builtin/pack-objects.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc sha1_file.c
Simple merge