refactor: indicate progress during branch checkout

This MR slightly improves the way extension indicate progress during checking out MR branch.

Before we just showed an information message (which stays displayed for roughly 10s).

Now we use vscode.window.withProgress() which wraps the git checkout promise and shows a progress bar for as long as the promise takes to resolve.

before	after
progress-before	progress-after

Related to #63 (closed)