Allow git:remote to update remotes with set-url

This changes the behavior of `heroku git:remote` when a remote with the same name already exists. Instead of just erroring out, it prompts the user to replace it. For non-interactive uses, the `--update` flag can be used to update without prompting.

cc: @dickeyxxx @friism @naaman @catsby @jsimone 
probably but, copied from `def create_git_remote(remote, url)` above...