Added Ctrl-D/Ctrl-U for Vim navigation in the Git graph


https://github.com/user-attachments/assets/8d3ad67c-829a-4a80-9508-80d48cf0decf



Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #55485 

Release Notes:
- Improved Vim navigation in the git graph with Ctrl-U and Ctrl-D
half-page scrolling.

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Co-authored-by: Anthony Eid <anthony@zed.dev>