In git, when you rewrite your history, the old version is gone (well, you still have the revlog for 30 days, but then it's basically over). This is why some commands like `git push --force`after a rebase can cause so much hassle to a community (hello Jenkins!)
Here, the principle is te keep all the history, and its rewrites, forever, and to ease the distribution of those changesets.
Nitpick: The old version will not be gone unless it is no longer reachable from any of the refs. Making it no longer reachable from merely one of many refs will not cause it to be GC'd.
This isn't really a nitpick though, since this means that similar porcelain could be implemented on top of git fairly easily. The underlying data-model supports it.
