Adding the `Update branch` to GitHub PRs
In my repositories, my PRs didn't have the Update branch button. This feature is nice when someone proposes a PR that breaks because of a separate issue. So I can fix that issue separately in a PR, merge it. Finally in the other person's PR, I can click Update branch, Rebase, then Rerun the workflows.

In order to show the Update branch option, go to the repository's Settings page, then check that box.

Now Update branch shows up.
