Personally it sems that number of commits is a metric too easy to game. If you generally are honest with yourself, keep it, but I wouldn't use it if I were to set a goal for a group of students. Another metric that is less easy to game on a personal level is time spent with your programming environment open, which is effective if you tend to either not start programming or stop prematurely. Finally the ideal metric is to have a set of features or a certain output you want to achieve and have that as a goal with the caveat that these goals tend to be too hard to achieve in the mean time.

So overall, I'd recommend time spent programming as a weekly goal and a final product as an overarching goal with the explicit option of re-negotiation.