Merge branch 'kn/maintenance-is-needed'

Merge branch 'kn/maintenance-is-needed'

"git maintenance" command learned "is-needed" subcommand to tell if
it is necessary to perform various maintenance tasks.

* kn/maintenance-is-needed:
  maintenance: add 'is-needed' subcommand
  maintenance: add checking logic in `pack_refs_condition()`
  refs: add a `optimize_required` field to `struct ref_storage_be`
  reftable/stack: add function to check if optimization is required
  reftable/stack: return stack segments directly