Disable hunk diff arrow buttons when there's only one hunk (#21437)

Closes https://github.com/zed-industries/zed/issues/20817

| One hunk | Multiple hunks |
|--------|--------|
| <img width="800" alt="Screenshot 2024-12-03 at 09 42 49"
src="https://github.com/user-attachments/assets/7c2ff80a-d4d9-4a74-84b8-891fadfd4e6c">
| <img width="800" alt="Screenshot 2024-12-02 at 23 36 38"
src="https://github.com/user-attachments/assets/60ea94b8-0b23-43a2-afad-b816b4645d1f">
|

Release Notes:

- Fixed showing prev/next hunk navigation buttons when there is only one
hunk