Creating a PR with changes on a protected branch fails silently

[sbatten]

Here are the steps I took:

1. I started in the editor window and created a session inside of the main branch of microsoft/vscode.
2. I then re-opened that session inside of the agents window.
3. This session is notably not backed by a work tree after I had changes that I wanted to bring into or submit for PR. I opened the changes view and clicked Create Pull Request.
   This fails because the changes have been committed to the main branch locally, which is not guarded because vs code is usually responsible for that. The agent doesn't care. Now it can't create a pull request because it can't push the active branch, so it silently fails.