git: Refine diff hunk controls visuals (#26317)

You may need to zoom in hard to see this 😅 but the main addition of this
PR is just ensuring there's also horizontal border instead of just on
the bottom. Also added some box-shadow here to make it pop out of the
diff a bit more.

| Before | After |
|--------|--------|
| ![CleanShot 2025-03-08 at 12  37
40@2x](https://github.com/user-attachments/assets/98e0329e-646f-4455-89fa-3f6ec1211361)
| ![CleanShot 2025-03-08 at 12  36
40@2x](https://github.com/user-attachments/assets/7f667e65-5b72-4156-b0ec-2b162eb76f2f)
|

Release Notes:

- N/A
6ed6e8bc26Danilo Leal committed on 3/8/2025, 4:00:47 AM· committed by GitHubparent4846e6f
1 file changedLine totals unavailable