Automatically switch to unified diffs when diff view is narrower than a configurable "minimum split diff width" (#52781)

Release Notes:

- The git diff diff view now automatically switches from split mode to
unified mode when the pane is narrower than a configurable minimum
column count. You can configure this via the new
`minimum_split_diff_width` setting.

crates/settings/src/vscode_import.rs

@@ -308,6 +308,7 @@ impl VsCodeSettings {
             completion_menu_scrollbar: None,
             completion_detail_alignment: None,
             diff_view_style: None,
+            minimum_split_diff_width: None,
         }
     }