I'm not the original asker, but evaluating programmers based upon the number of commits per day is a lot like evaluating them based on the number of lines of code written per day -- it's a measure of activity, rather than a measure of productivity, and those often aren't the same thing.

(I'm not trying to imply that that's a mistake you've made; it's just something that it's easy to forget as a manager of programmers, and is maybe worth pointing out explicitly; you need a way to measure the team's productivity, but it's very hard to find a good, measurable proxy for 'productivity' which isn't eventually going to end up being gamed by the whole team!)