In VSCode Extension, Open File in Sourcegraph Web shows a 404 if current branch does not exist and defaultBranch is set

Created by: wukkuan

• Sourcegraph version: Sourcegraph.com (affects all versions)
• Platform information: macOS, vscode extension 2.2.3

Steps to reproduce:

1. Install sourcegraph vscode extension
2. Set sourcegraph.defaultBranch to master in vscode settings
3. Clone https://github.com/wukkuan/sourcegraph-test
4. Check out a new branch that doesn't exist (name doesn't matter, but let's go with test)
5. Open the cloned repository in vscode
6. Open the README.md file
7. Right click in the file, click Open file in sourcegraph web

Expected behavior:

The branch does not exist on the remote, so we expect to be directed to the file on sourcegraph.com on the master branch. The URL should be similar to: https://sourcegraph.com/github.com/wukkuan/sourcegraph-test@master/-/blob/README.md

Actual behavior:

We are directed to the file on sourcegraph.com on the main branch (which does not exist) and see a 404 error. The URL is similar to: https://sourcegraph.com/github.com/wukkuan/sourcegraph-test@test/-/blob/README.md

If I remove the sourcegraph.defaultBranch setting I am prompted to choose between "main" and cancelling.