If a delegate is missing the default branch for a project certain
computations will fail, e.g. getting the canonical mainline branch.
One prevention for corrupting data is refusing to fetch a delegate
that does not have the default branch, when the repository is a
project. It is assumed that the repository is not a project if calling
`Doc::project` returns an error, since the error variants are not
found and parsing errors.
This is only checked in the case of a delegate since non-delegates are
safe to created COBs that don't require the default branch,
e.g. creating an issue.
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