The oldest format of repository does not have requirements so we need to treat
them differently when cloning such repository.
The previous code had issue whenever we start using a working-copy-only
requirements. The "legacy" format is signaled using an empty requirements list.
If we add working-copy-only requirements to it, this is no longer empty, and
no longer legacy.
The new code fix this, and will get fully tested in a couple of changeset, once the
share-safe become the default.