Who said it's supposed to be an abstraction? The point, theoretically, of something like Git is that the actual unvarnished model is clear enough that you don't need an abstraction. The problem IMO is that the commands are kind of random and don't map cleanly to the model.
There are couple projects that try to tackle this problem by providing an alternative CLI (on top of git's own plumbing), like gitless and g2. Haven't used any of them myself, but would be interested in experience of others.
