PR file list keeps the old base after the base branch changes outside VS Code #9076

• Extension version: 0.170.0
• VSCode Version: 1.142.0
• OS: Windows 11
• Repository Clone Configuration (single repository/fork of an upstream repository): single repository
• GitHub Product (GitHub.com/GitHub Enterprise version x.x.x): GitHub.com

Steps to Reproduce:

1. Have an open PR listed in the Pull Requests view.
2. Change its base branch outside VS Code, for example with gh pr edit <number> --base <branch>, to a branch where the PR's files differ.
3. Expand the PR in the Pull Requests view, then click Refresh and expand it again.

Result: the file list still compares against the old base. In my case, files that the new base already has as renamed show as deleted and added. Only Developer: Reload Window fixes it. GitHub's "Files changed" tab shows them as modified.

Expected: Refresh shows the files against the new base.

From reading dist/extension.js (not debugged): pr.refreshList calls forceClearCache(), and createOrUpdatePullRequestModel() keeps the existing model and calls update(), which sets the new base. Nothing clears _fileChanges, and getChangeModels() reuses it while it isn't empty.