The actual problem is one of power and intention. Yes, version control would make earmarks obvious. Yes, it would make tracking contributors (lobbies) easier. Yes, it would make tailoring tried and trusted legal documents easy.
All of these are reasons why version control will never be applied to the law. We want these things, but we are not the customer. The real customer is actively trying to prevent these things from ever happening.
The reality is that these tools are probably already being used for these exact purposes - in private, and for personal gain.
Of course we won't see Congress outlaw lobbying, but there's definitely an opportunity today to try some of these concepts in an organization, community, or even willing local government.
/rant
Continuing with OP's thought. Making the legislator who introduces the bill commit the bill would be an awesome piece of information and might make some legislators think twice about what they introduce and how they title it. And all the statistics that could be run on a repo of laws would be a ridiculously useful resource.
I even registered gitlaw.us a few years ago and then let it expire because I couldn't bridge the impedance mismatch between the idea and the implementation.
But again, I would never voice such thoughts, for many reasons:
1. It's exactly the attitude that your 'real customers' want in the public - learned helplessness.
2. You start believing it, and there is an insidious, negative effect on your judgement, decisions, and life. I'm not saying be a rose-tinted-glasses optimist, but don't be the opposite extreme either.
3. It is possible to win. Remember Gandi, Mandela, etc. Imagine how bleak their outlook was, yet they remained committed, steadfast, relentless, constant as the dawn, and eventually the time came for their ideas and there was nothing more powerful or unstoppable.
4. Real hackers aren't like that. The bigger the problem, the bigger the opportunity for disruption and making the world better. There are few bigger than this.
It takes efforts like these to eventually make the time come for the ideas that change the world, so it behooves us all never to talk them down out of mere pessimism. Constructively criticize yes, but no shootdowns.
Remember, first they ignore you, then they ridicule you, then they fight you, then you win.
It can be done.
So make it happen first, then make it work well.
The problem is that the government is too big and has too much power, so the rewards for lobbying are so great.
It's not cynicism, its realism which must be reflected in the strategy for change, otherwise failure is inevitable.
However, in the 200+ year history of the United States Congress, no tool like this has existed that could bring the kind of transparency to the process that reformers have been pushing for for nearly as long.
It is one thing to say "We need more transparency!" It is another thing entirely to say "Publish all bill modifications with attribution using a public version control system!" Having something specific and concrete to push for, something the implementation of which can be said to be completed or not completed in definitive terms, can make the difference between a successful push for reform and an unsuccessful one.
The reality is that most of the people who complain about the lack of transparency are those who put the least effort into finding the available sources.
A separate problem, however, is that this is essentially a process improvement, that doesn't generate much advantage to the person who makes it happen. A politician would need to use significant personal resources to push this type of reform through the system, yet the 'payoff' is probably minimal compared to substantive policy or constituent service activities. By raising the issue from process improvement to substantive issue, the chances of implementation improve. Discussing and spreading the idea is valuable in that light.
On a side note, once these types of distributed systems begin to take root, it becomes quickly apparent that co-location in Washington is of at least questionable value. (Look at the attendance records of members at Congressional hearings.) Virtually everything that Congress does can be accomplished without the need for the massive physical and personnel support system found at the Capitol. It would be interesting to see how government would be designed given the communication tools available today.