Suppress output during git index refresh in syncback

- Updated the `refresh_git_index` function to redirect both stdout and stderr to null, preventing unnecessary output during the execution of the `git update-index --refresh` command. This change enhances the user experience by reducing clutter in the console logs.

src/cli/syncback.rs

@@ -366,6 +366,8 @@ fn refresh_git_index(project_dir: &Path) {
         match Command::new("git")
             .args(["update-index", "--refresh", "-q"])
             .current_dir(project_dir)
+            .stdout(Stdio::null())
+            .stderr(Stdio::null())
             .status()
         {
             Ok(_) => log::info!("Git index refreshed."),