I came up with 19,290 using the generalized birthday formula[0] (actually after double-checking it's slightly closer to 19,291).
16,384 is the value you get using the square approximation method[1] which I believe is a bit less accurate in terms of probability, but faster to calculate. I think Git's using square approximation under the hood -- which is probably a good thing since I think it'll always yield a more conservative result.