Add tag modification detection

When synchronizing Git repositories, tags are force-pushed on the destination Gitlab. This is required for partial Git tags (e.g. 6 or 6.1) since they are meant to be updated but this should not be for complete Git tags (e.g. 6.1.3).

Moreover if a complete Git tag has been modified since the last synchronization this could mean an attacker is trying to fool users to use a malicious commit (cf https://www.stepsecurity.io/blog/harden-runner-detection-tj-actions-changed-files-action-is-compromised).

Should gitlab-cp stop the synchronization and fail when it detects a complete tag has been modified?