They are closer semantics as 'html_url' is the main page of the repository,
so it is the best to identify it; and 'clone_url' is the URL that should
be given to 'git clone', as documented by https://schema.org/codeRepository
Additionally, that property was missing so far; but a future commit will
need to use it to identify fork relationships (node ids are required to
representation relationships between documents as we cannot use blank
nodes for that)
Depends on D8460.