Add advice to view existing branches in pr_resume()

[olivroy]

Let's say I call pr_resume("git"), but branch git2 exists. pr_resume() will error but there is no hint that branch git2 exists already.

This adds a runnable link to view local branches

[jennybc]

pr_resume() (called with no input) already has a pretty nice experience, in terms of offering choices. I view that as the solution to "Hmmm, I don't know the exact name of the branch I want to checkout".