chore: Fix several style lints (#17488)

It's not comprehensive enough to start linting on `style` group, but
hey, it's a start.

Release Notes:

- N/A

crates/git/src/diff.rs

@@ -63,6 +63,12 @@ pub struct BufferDiff {
     tree: SumTree<DiffHunk<Anchor>>,
 }
 
+impl Default for BufferDiff {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl BufferDiff {
     pub fn new() -> BufferDiff {
         BufferDiff {