Git extension API branch `upstream` is undefined when repo was cloned shallow

Description

When cloning a repository with git clone --depth 1, creating a new branch, and then attempting to create a pull request using the VSCode GitHub Pull Request extension, the following issues occur:

1. The AI-powered PR title and description generator does not work.
2. After creating the PR, the extension does not detect it properly.

---

You can post this on the [GitHub Pull Request extension's issue tracker](https://github.com/microsoft/vscode-pull-request-github/issues). Let me know if you need any modifications! 🚀

• Extension version: 0.106.0
• VSCode Version: 1.96.4
• Git Version: 2.43.0
• OS: macos sonoma
• Repository Clone Configuration (single repository/fork of an upstream repository): fork of an upstream repository
• Github Product (Github.com/Github Enterprise version x.x.x): Github.com

Steps to Reproduce

1. Clone a repository with git clone --depth 1 https://github.com/hichemfantar/nextra-fork.
2. Create and switch to a new branch:

   git checkout -b new-feature
   

3. Make some changes and commit them.
4. Open the GitHub Pull Request extension and attempt to create a PR.

Expected Behavior

• The AI-powered PR title and description should generate properly.
• The created PR should be detected by the extension.

Actual Behavior

• The AI-powered PR title and description generator does not work.
• The extension does not detect the created PR after it's created.

Additional Notes

• The issue may be caused by the shallow clone preventing the extension from accessing full branch history.