Revert: Spawn terminal process on background executor (#41060)

Reverts https://github.com/zed-industries/zed/pull/40774 and
https://github.com/zed-industries/zed/pull/40824 since they introduce a
bug where Nushell processes are leaked and Ctrl+C doesn't kill the
current process.

Release Notes:

- Fix a bug where nushell processes wouldn't get killed after closing a
terminal tab

crates/git_ui/src/project_diff.rs

@@ -640,16 +640,12 @@ impl Item for ProjectDiff {
         _workspace_id: Option<workspace::WorkspaceId>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Task<Option<Entity<Self>>>
+    ) -> Option<Entity<Self>>
     where
         Self: Sized,
     {
-        let Some(workspace) = self.workspace.upgrade() else {
-            return Task::ready(None);
-        };
-        Task::ready(Some(cx.new(|cx| {
-            ProjectDiff::new(self.project.clone(), workspace, window, cx)
-        })))
+        let workspace = self.workspace.upgrade()?;
+        Some(cx.new(|cx| ProjectDiff::new(self.project.clone(), workspace, window, cx)))
     }
 
     fn is_dirty(&self, cx: &App) -> bool {