/diff for session changes only instead of `git diff`

### Describe the feature or problem you'd like to solve

I want to see the changes from the session, not the `git diff`. The current /diff command doesn't add much value; I can run the `git diff` command myself and see the same info..

### Proposed solution

`/diff session` or `/session diff` command to view only the edits copilot has made during the session. Based on the `/session` command, it looks like this info is already being tracked, but I don't see a way to see the changes.

### Example prompts or workflows

I have some changes in my working set, start copilot, and it makes some edits. I want to see what it did and what I already had done.

### Additional context

_No response_