Merge branch 'ps/submodule-ref-format'

Merge branch 'ps/submodule-ref-format'

Support to specify ref backend for submodules has been enhanced.

* ps/submodule-ref-format:
  object: fix leaking packfiles when closing object store
  submodule: fix leaking seen submodule names
  submodule: fix leaking fetch tasks
  builtin/submodule: allow "add" to use different ref storage format
  refs: fix ref storage format for submodule ref stores
  builtin/clone: propagate ref storage format to submodules
  builtin/submodule: allow cloning with different ref storage format
  git-submodule.sh: break overly long command lines