`keybase prove twitter` canceled in the middle of process, prompted with "You already have a proved you are mbrownnnyc at twitter; overwrite?"

Hello guys,

I canceled the `keybase prove twitter` command at one of the internal prompts, upgraded `keybase`, then ran the `keybase prove twitter` command again, and was prompted with "You already have a proved you are mbrownnnyc at twitter; overwrite?"

Seems trivial, but somethin' ain't right about that conditional.

Start by reproducing `keybase prove twitter`, cancel it at an internal prompt, and run the command again. Trace the state used for the "already have a proof" prompt and confirm that a canceled run does not leave an existing proof; done means the second run no longer incorrectly offers to overwrite an incomplete proof.

Written by the indexing model from the issue text.