Merge branch 'kn/update-ref-symref'

Merge branch 'kn/update-ref-symref'

"git update-ref --stdin" learned to handle transactional updates of
symbolic-refs.

* kn/update-ref-symref:
  update-ref: add support for 'symref-update' command
  reftable: pick either 'oid' or 'target' for new updates
  update-ref: add support for 'symref-create' command
  update-ref: add support for 'symref-delete' command
  update-ref: add support for 'symref-verify' command
  refs: specify error for regular refs with `old_target`
  refs: create and use `ref_update_expects_existing_old_ref()`