project: Spawn terminal process on background executor (#40774)

We were spawning the process on the foreground thread before which can
block an arbitrary amount of time. Likewise we no longer block
deserialization on the terminal loading.

Release Notes:

- Improved startup time on systems with slow process spawning
capabilities

crates/git_ui/src/project_diff.rs

@@ -625,12 +625,16 @@ impl Item for ProjectDiff {
         _workspace_id: Option<workspace::WorkspaceId>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Entity<Self>>
+    ) -> Task<Option<Entity<Self>>>
     where
         Self: Sized,
     {
-        let workspace = self.workspace.upgrade()?;
-        Some(cx.new(|cx| ProjectDiff::new(self.project.clone(), workspace, window, cx)))
+        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)
+        })))
     }
 
     fn is_dirty(&self, cx: &App) -> bool {