It looks like an estimation, not a VCS link.

Think of it this way: Vim undo history is a tree which you can walk visiting every branch (not that it's a thing you want to do). Now, writing all this data out has some cost in IO bandwidth - comparable to bandwidth of the keyboard, i.e. kBytes/minute. Vim users don't notice the cost of maintaining the tree in RAM.

Synchronising it at first opportunity is also not hard if you do it in the background and so latency can be tolerated most of the time.

The merges.. you can try to do them mostly on marked commits, and then they can be done just like they are done now.

But implementing all that is a great undertaking, to be sure.