Although it may have only taken a week to create the initial version of Git the amount of polish and documentation required to get it to something someone like me who's not a kernel developer was a lot more than a week's worth of work. You may be able to get 80% of the functionality in 20% of the time but it's that last 20% of functionality that gives software enough polish to actually be used by someone who doesn't have an incredibly specific reason to figure out how to use the unpolished software.