This PR adds the recently introduced "branch diff" mention option to the
"Add Context" menu in the message editor:

<img width="450" height="582" alt="Screenshot 2026-03-13 at 9  58@2x"
src="https://github.com/user-attachments/assets/1f5102e7-9018-4a5b-992d-43befa55dcbe"
/>

Release Notes:

- N/A