Ephemeral repos created by open_repo don't include objects from turnip-subordinate

[William Grant]

When turnip.api.store.open_repo constructs an ephemeral repo using alternates, it expects that the alternates' alternates will be respected. But Git and libgit2 don't respect relative alternates except in the root repo (with no clear rationale), so repos with a turnip-subordinate alternate will be missing objects.

It's easiest to work around this by making open_repo explicitly include each repo's turnip-subordinate.