Add ability to update PRs with the default branch (i.e. The Github PR "Update Branch" feature)

[parthea]

Is your feature request related to a problem? Please describe.
It's tedious when I have to manually click the "Update Branch" button on PRs in order to merge the code from the default branch into my branch.
Describe the solution you'd like
It would be great if the repo tool could support updating branches with the main branch. For example, using repo update or repo update-branch
Describe alternatives you've considered
Right now I have a workaround which is to use selenium to click the 'Update branch' button on PRs where I'd like to merge the main branch into my branch.

[alexander-fenster]

As a workaround for this, repo tag ... automerge will trigger the merge-on-green bot that will update branch before merging.