release-init-request.json missing source_commit_hash

run

```
go run github.com/googleapis/librarian/cmd/librarian release init -repo https://github.com/ldetmer/google-cloud-python -push
```

the request-release-init.json is missing source_commit_hash field
https://paste.googleplex.com/5640041330245632

