Add /diff to the assistant

[ross39]

Release Notes:

• Closes Add /diff to the assistant #16821 (I think? Maybe more needs to be done?)

Not sure if this fully closes #16821 but a basic git diff has been added to the assistant panel. As this is my first PR and I am new, any feedback would be appreciated. I am happy to make this an extension if the zed team thinks an extension would be a better place for this. Thanks! 👋

Below are screenshots of what it looks like in the assistant panel.

How the command looks in the assistant panel

How the diff looks when added to the context

[zed-industries-bot]

	Warnings
⚠️	This PR is missing release notes. Please add a "Release Notes" section that describes the change: Release Notes: - Added/Fixed/Improved ... If your change is not user-facing, you can use "N/A" for the entry: Release Notes: - N/A

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