[Bug]: Diff staged-vs-working split missing for Markdown files

### Problem or use case

When the same file in the git repository was edited, staged (`git add`), then edited again, there will be two entries of the file in Source Control pane. One under "Changes", another - "Staged changes".

Currently Changes show the whole diff comparing to branch state, so there's effectively no way to see diff since last `git add`. Personally I use it a lot while working with the agent iteratively and staging changes that I've reviewed, so I don't have to review everything on each interation.

### Proposed solution

The expected behaviour for me would be "Staged changes" to show diff between commited state and staged, while "Changes" to show diff between staged and current file state. That's how VS Code shows it for example.

### Alternatives or additional context

Thanks for the great product!