Issue Type: Bug

In review mode, clicking on any file in "changes in pull request" at all will yield the error "We couldn't find commit", and the diff will be empty or just contain the full text of the remote file.

to reproduce:

1. open a project with no submodules
2. check out a pull request locally, verify that the review mode generally works
3. close vscode
4. use git submodule add to add a submodule. (e.g.: make a local checkout of a different git repo, then in the original one, git submodule add ../path/to/checkout submodule1)
5. open vscode again
6. Github Pull Request extension is now confused and not showing any PRs
7. navigate to Source Control and make sure only the original repository, not the submodule, is selected
8. go back to Github Pull Request extension, see that PRs are visible again
9. in "changes in pull request" (should be available from last time), click on any file. The "we couldn't find commit" message will pop up and the display will be wrong.

The commits in question exist (as can be verified by git in the command line), but the "Git" output window shows errors on them. Presumably the commands are called on the wrong repository.

The problem affects the stable version of the extension as well.

Extension version: 2020.2.8161
VS Code version: Code 1.42.1 (c47d83b293181d9be64f27ff093689e8e7aed054, 2020-02-11T14:50:36.977Z)
OS version: Linux x64 4.15.0-88-generic