Finally, update PullDestination to use ShortRepository

While re-recording cassettes I discovered a couple tiny bugs

Fix-up some tests so I can re-record them as well

It isn't exactly correct, but it's better than compromising on the
design of Repository.

So far we have two Repository objects (ShortRepository, Repository). In
looking at API responses, it seems GitHub has removed many Repository
attributes altogether. I'll need to investigate this further.

Related-to: gh-670

Split User object into more accurate objects

They removed total_private_gists. For now, I'm opting to totally remove
it. We may add it back later as a perpetual "None" though.