On first run in a git repository, `BaseRepo()` will now prompt the user
which repository should be queried as base repository if there are
multiple git remotes or when we are in the context of a fork.
In non-interactive mode, the prompt is skipped and we default to the
first git remote instead.
After the base repo is resolved, the result is cached in the local
repository using `git config` so that RepositoryNetwork API lookups can
be avoided in the future.
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