Source Control - Git - Show submodule diffs properly, or at least not an error

When clicking on a submodule diff from the diff or graph view, you get this:

The logs show:
```
[error] [Window] Error: Unable to read file 'git: ...'
```

Given this shows as an error, I'm counting it as a bug.
Ideally it could show something like what github has: https://github.com/hudson-trading/slang-server/pull/500/changes, and perhaps some info about the sub commits.

- VS Code Version: 1.133.0

Steps to Reproduce:
Open a commit with a git submodule change in the scm graph; click on the submodule change.