Function to do the remote rename dance

[jimhester]

When people first start contributing to a package one of the first major roadblocks is they likely have cloned the repo from the initial repo rather than from a fork. So they have to

1. rename the origin remote to upstream then add the new remote as origin
2. or burn it all down and re-clone from their fork, then add the origin as upstream later.

As far as I know there is no real way to do either step without needing command line git or something like Sourcetree, so it ends up being a pretty big hurdle for new contributors.

[hadley]

I think we should probably test for this situation in pr_init(), and if in an interactive environment, offer to resolve it automatically.

[jennybc]

use_git_remote() now makes this possible within R and the "remote rename dance" is one of the examples. It's possible we'll even provide more support in a hypothetical "unbork my fork" function (#618 Undo commits to a fork's master branch).