scap backport: Enhance handling of checked-out but not live branches

Problem: Previously, scap backport encountered key_error with branches checked out for deployment but not yet live in production. As mentioned in the phabricator task T317795

Solution: The program now fetches the commit fingerprints for backported change and syncs live branch commits and it will prompt user to backport the changes.

Example: If scap backport request is for branch 'wmf/php-1.42.0-wmf.19' and live branch is 'wmf/php-1.42.0-wmf.18', so the code change will identify 'wmf/1.42.0-wmf.19' and it will fetch the commit fingerprint for'wmf/1.42.0-wmf.19' along with 'wmf/1.42.0-wmf.18'.

Bug: T317795