Rebootstrap pipeline should check for existing PRs and update them.

It would be nice if the rebootstrap pipeline had the ability to update existing rebootstrap PRs. Currently it always opens new PRs. The downside to this is you have to close/link existing PRs and you loose the context/conversation history in the new PR. New PRs are also an inconvenience for subscribers waiting for notifications on when the rebootstrap is complete. Having to rebootstrap several times as you work through issues is fairly common in previews.