This PR removes the commit message from the first excerpt of the
multibuffer and renders it as markdown in the commit view's header.
Effectively, this allows the commit view to use the split diff buffer,
as done in https://github.com/zed-industries/zed/pull/58163.

<img width="600" alt="Screenshot 2026-06-01 at 6  34@2x"
src="https://github.com/user-attachments/assets/a763a04d-88ce-4d29-9cee-872a33ace6e4"
/>

Release Notes:

- N/A