agent_ui: Insert branch diff crease when clicking on menu item (#51509)

Follow up to https://github.com/zed-industries/zed/pull/51487

The PR above added the item to the menu, and this one makes the menu
item actually insert a mention crease with the branch diff. That was
missing in the previous one.

Release Notes:

- N/A
a623dc3d1aDanilo Leal committed on 3/13/2026, 5:13:04 PM· committed by GitHubparentf04b4e0
3 files changedLine totals unavailable