Initial git panel refinements (#21912)

- Wire up settings
- Update static Panel impl
- Tidy up renders

Release Notes:

- N/A

crates/git_ui/src/git_ui.rs

@@ -1 +1,10 @@
+use ::settings::Settings;
+use gpui::AppContext;
+use settings::GitPanelSettings;
+
 pub mod git_panel;
+mod settings;
+
+pub fn init(cx: &mut AppContext) {
+    GitPanelSettings::register(cx);
+}