New excerpt controls (#24428)

Release Notes:

- Multibuffers now use less vertical space for excerpt boundaries.
Additionally the expand up/down arrows are hidden at the start and end
of the buffers

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Zed AI <claude-3.5-sonnet@zed.dev>

crates/git_ui/src/git_panel.rs

@@ -358,7 +358,6 @@ pub(crate) fn commit_message_editor(
         EditorMode::AutoHeight { max_lines },
         buffer,
         None,
-        false,
         window,
         cx,
     );

crates/git_ui/src/project_diff.rs

@@ -141,13 +141,8 @@ impl ProjectDiff {
         let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
 
         let editor = cx.new(|cx| {
-            let mut diff_display_editor = Editor::for_multibuffer(
-                multibuffer.clone(),
-                Some(project.clone()),
-                true,
-                window,
-                cx,
-            );
+            let mut diff_display_editor =
+                Editor::for_multibuffer(multibuffer.clone(), Some(project.clone()), window, cx);
             diff_display_editor.disable_inline_diagnostics();
             diff_display_editor.set_expand_all_diff_hunks(cx);
             diff_display_editor.register_addon(GitPanelAddon {