Describe the feature or problem you’d like to solve

The command gh repo clone OWNER/REPO requires me to specify my username.

Proposed solution

The command gh repo clone REPO should default OWNER to the user when the <repository> argument doesn't have a /. The hub cli works this way, and it's convenient.

Additional context

When gh repo clone was added, #333 (comment) asked for the default OWNER to respect git's config, which could override this default if a gh.owner value is set. I think that's interesting, but out of scope for this FR.