editor: Fix diff hunk controls not shown until buffer interaction (#32692)

Similar to https://github.com/zed-industries/zed/pull/32683, checking
mouse hovered in `mouse_move` instead of `prepaint` for diff hunk
controls.

Release Notes:

- Fixed issue where diff hunk controls were not visible on mouse hover
when `cursor_blink` is `false`.

crates/editor/src/editor.rs

@@ -1088,6 +1088,7 @@ pub struct Editor {
     tasks_update_task: Option<Task<()>>,
     breakpoint_store: Option<Entity<BreakpointStore>>,
     gutter_breakpoint_indicator: (Option<PhantomBreakpointIndicator>, Option<Task<()>>),
+    hovered_diff_hunk_row: Option<DisplayRow>,
     pull_diagnostics_task: Task<()>,
     in_project_search: bool,
     previous_search_ranges: Option<Arc<[Range<Anchor>]>>,
@@ -2016,6 +2017,7 @@ impl Editor {
 
             breakpoint_store,
             gutter_breakpoint_indicator: (None, None),
+            hovered_diff_hunk_row: None,
             _subscriptions: vec![
                 cx.observe(&buffer, Self::on_buffer_changed),
                 cx.subscribe_in(&buffer, window, Self::on_buffer_event),