Option of syncing data when redoing an upgrade

[nmeirik]

The process of doing an upgrade usually consists of:

1. Upgrade the site
2. Test the site
3. Lock the site for editing
4. Upgrade the site again (to get the new data which has been created during the testing)
5. Open site for editing

For this forth step, it should not be neccessary to perform copying of the entire site (at least not extensions and entire var/ directory again) since little has been changed. Instead an rsync should be performed. This will greatly improve the speed ot the second (live) upgrade and result in less time needed to lock down the site while performing the upgrade.