Three problems, not picked up while reviewing this MR:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58677

First, the `args` argument to `resolver_complexity` doesn't implement
`fetch`, so any queries are currently broken.

Second, `::Blob` type doesn't implement a couple of fields found in
`TreeEntry`, and `BlobType` depends on them. So if you query for them
from the new `Blob` endpoint, the query fails.

Third, sharing the GraphQL `Blob` type between the two endpoints is not
practical.

Fix all three problems before a release, and introduce a feature spec
to prevent the first two from recurring.