Rebasing isn't really destructive. Try it: open `gitk --all`, do a rebase -i and squash all the commits together, then switch back to gitk and refresh. You'll see all the old commits still in the database, but the HEAD and master branches point at the new commits. Totally not destructive, but it does take a bit of knowledge to undo the change.

Git maintains what's called a "reflog" to help with this. It's a record of every place that every ref has ever pointed in your repository. Try it: `git reflog master`. You can even see when you did pulls, and what they merged in.

The only truly destructive thing you can do to your git database is `git gc`.