git_ui: Branch picker improvements (#26287)

- Truncate branch names based on the width of the picker
- Use a footer for "Create branch" instead of a picker entry

Still to do:

- [x] Select the footer button when no matches and run the create logic
on `enter`
- [x] Make it possible to quickly select the footer button from the
keyboard when there are matches

Release Notes:

- Git Beta: Removed limitation that made it impossible to create a
branch from the branch picker when it too closely resembled an existing
branch name

crates/tab_switcher/src/tab_switcher.rs

@@ -325,8 +325,8 @@ impl PickerDelegate for TabSwitcherDelegate {
         Arc::default()
     }
 
-    fn no_matches_text(&self, _window: &mut Window, _cx: &mut App) -> SharedString {
-        "No tabs".into()
+    fn no_matches_text(&self, _window: &mut Window, _cx: &mut App) -> Option<SharedString> {
+        Some("No tabs".into())
     }
 
     fn match_count(&self) -> usize {