I've put together another write-up of the Git 2.11 release that discusses some of the other new features (and goes into a little more detail on some of the 'sundries'): https://medium.com/@kannonboy/whats-new-in-git-2-11-64860aea...
I'm also curious how you came up with 19,290 for a birthday paradox on a 7-hex hash. I think it's 16,384, but probability can sometimes be tricky. :)
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.
[0]: https://en.wikipedia.org/wiki/Birthday_problem#Cast_as_a_col...
[1]: https://en.wikipedia.org/wiki/Birthday_problem#Square_approx...