Tab completion for repo names in hub clone

[apjanke]

How about some tab completion for the shorthand repo names in the hub clone command?

This PR adds support for tab completion of repo names in git clone <repo>. It supports both un-qualified repo names for the current user, and <user>/<repo> qualified names.

Examples:

hub clone <TAB>

will list all the current user's own repos.

hub clone foo<TAB>

will complete this user's repos starting with "foo".

hub clone defunkt/<TAB>

will list all repos for user defunkt.

Limitations:

Does not complete user names, because there's just too darn many of them. (Maybe if the GitHub API exposed a prefix-filtered user listing endpoint, we could do that too.)

It's pretty slow right now. If this PR is accepted, I'll add some caching to it.

[mislav]

@apjanke There is a hub api command now for low-level API access! You could consider re-implementing this using GraphQL calls (they're going to be faster) instead of adding a new internal endpoint. https://github.com/github/hub/releases/tag/v2.8.3 #2016

Ref. #871

[apjanke]

Yep, that's a good idea. Don't pull this; I'll rewrite.