Improve git-status performance

Followup to pull request #36 which got closed by dropping the devel branch.

The core.preloadindex improvements resulting from @pro-logic's suggestions have already been merged to junio/next [1].

[1] https://github.com/msysgit/git/commit/7349afd2

I'm not entirely happy yet with how these patches integrate with existing mingw / dirent code, so expect another reroll when I find the time to polish things up. In the meantime, I'd love to get reviews and / or test results. See pull request #46 for a more fine-grained way to test performance of git commands.