git: Amend (#28187)

Adds git amend support.

- [x] Turn existing commit button into split button
- [x] Clean up + Handle shortcuts/focus cases
- [x] Test remote

Release Notes:

- Added git amend support.

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/ui/src/components/button/split_button.rs

@@ -20,6 +20,12 @@ pub struct SplitButton {
     pub right: AnyElement,
 }
 
+impl SplitButton {
+    pub fn new(left: ButtonLike, right: AnyElement) -> Self {
+        Self { left, right }
+    }
+}
+
 impl RenderOnce for SplitButton {
     fn render(self, _window: &mut Window, cx: &mut App) -> impl IntoElement {
         h_flex()