When cloning a forked repository, `gh repo clone` automatically adds the
parent repo as an `upstream` remote and sets it as the default repository.
This can be problematic when the user lacks access to the parent repo,
the upstream fetch is expensive for large repos, or the user simply
doesn't want the upstream remote.
Add a `--no-upstream` flag that skips adding the upstream remote when
cloning a fork. When used, origin (the fork) is set as the default
repository instead. The flag is mutually exclusive with
`--upstream-remote-name`. For non-fork repos the flag is a harmless
no-op.
Closes#8274
fa95f3a21b