Change the default staging and unstaging state display (#26299)

This adds a setting for the "border" hunk display mode, as discussed,
and makes it the default.

Here's how it looks in light mode:

<img width="1512" alt="Screenshot 2025-03-07 at 11 39 25 AM"
src="https://github.com/user-attachments/assets/a934faa3-ec69-47e1-ad46-535e48b98e9f"
/>

And dark mode: 

<img width="1511" alt="Screenshot 2025-03-07 at 11 39 56 AM"
src="https://github.com/user-attachments/assets/43c9afd1-22bb-4bd8-96ce-82702a6cbc80"
/>


Release Notes:

- Git Beta: Adjusted the default hunk styling for staged and unstaged
changes

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Nate <nate@zed.dev>

crates/gpui/src/color.rs

@@ -634,7 +634,7 @@ impl Display for ColorSpace {
 }
 
 /// A background color, which can be either a solid color or a linear gradient.
-#[derive(Debug, Clone, Copy, PartialEq)]
+#[derive(Clone, Copy, PartialEq)]
 #[repr(C)]
 pub struct Background {
     pub(crate) tag: BackgroundTag,
@@ -646,6 +646,28 @@ pub struct Background {
     pad: u32,
 }
 
+impl std::fmt::Debug for Background {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        match self.tag {
+            BackgroundTag::Solid => write!(f, "Solid({:?})", self.solid),
+            BackgroundTag::LinearGradient => {
+                write!(
+                    f,
+                    "LinearGradient({}, {:?}, {:?})",
+                    self.gradient_angle_or_pattern_height, self.colors[0], self.colors[1]
+                )
+            }
+            BackgroundTag::PatternSlash => {
+                write!(
+                    f,
+                    "PatternSlash({:?}, {})",
+                    self.solid, self.gradient_angle_or_pattern_height
+                )
+            }
+        }
+    }
+}
+
 impl Eq for Background {}
 impl Default for Background {
     fn default() -> Self {