manifest: add shallow option to treemanifestctx.readdelta and readfast The old manifest had different functions for performing shallow reads, shallow readdeltas, and shallow readfasts. Since a lot of the code is duplicate (and since those functions don't make sense on a normal manifestctx), let's unify them into flags on the existing readdelta and readfast functions. A future diff will change consumers of these functions to use the manifestctx versions and will delete the old apis.