Better completions for repos, issues, PRs, etc. in commands

Editing to clarify for people who come upon this issue that:

- Shell completion of commands is already supported in gh
- This issue is about shell completion for particular GitHub objects (for example, easily cycling through possible PRs to checkout or view)
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Original issue content:

It would be great to have completions for different shell environments. Not only for commands, but, for example, cycling through open pull request when checking out a PR branch or something like that.