Merge branch 'tb/stdin-packs-excluded-but-open'

Merge branch 'tb/stdin-packs-excluded-but-open'

pack-objects's --stdin-packs=follow mode learns to handle
excluded-but-open packs.

* tb/stdin-packs-excluded-but-open:
  repack: mark non-MIDX packs above the split as excluded-open
  pack-objects: support excluded-open packs with --stdin-packs
  t7704: demonstrate failure with once-cruft objects above the geometric split
  pack-objects: refactor `read_packs_list_from_stdin()` to use `strmap`
  pack-objects: plug leak in `read_stdin_packs()`