Update rendering of gutter diff hunks to show whether a hunk is staged or not (#26809)

In the gutter, it seems more intuitive to me for the unstaged hunks to
be hollow, indicating an action left to complete, and the staged hunks
to be filled. I therefore flipped the style of expanded hunks to match
the gutter icons. Is that acceptable? And would it be a breaking change?
If it is not acceptable, then 058dc216d5a284033eebd38d28853f69aa336a55
contains the opposite behaviour, it is not a problem to revert to it.

In the following images, the first hunk is always ~unstaged~ staged and
the second is ~staged~ unstaged.

<img width="138" alt="image"
src="https://github.com/user-attachments/assets/35927069-da90-424a-8988-a4eb984d865f"
/>
<img width="133" alt="image"
src="https://github.com/user-attachments/assets/4edd0e0d-a2b5-453a-8172-47684e065c82"
/>

<br />
<img width="143" alt="image"
src="https://github.com/user-attachments/assets/2f295944-81aa-45f3-a103-c13b92bc2aba"
/>
<img width="133" alt="image"
src="https://github.com/user-attachments/assets/35248218-7104-4059-8742-ae0e54da6c6b"
/>


Release Notes:

- Improved gutter diff hunks to show whether a hunk is staged