Fix: skip redundant git checkout when branch is already checked out

When checkout() is called for the current branch, skip the git
checkout command while still running the full operation lifecycle
(updating model state and firing onDidCheckout events). This allows
extensions like GitHub Pull Requests to safely call checkout()
unconditionally without needing their own 'already on branch' guard.

Agent-Logs-Url: https://github.com/microsoft/vscode/sessions/86a07518-2865-4107-8627-d5a4da7535cb

Co-authored-by: alexr00 <[email protected]>