Another fix for PATH and msysgit.

Evan Shaw tells me the previous fix didn't work. Let's use this one instead, which he says does work. This fix is kind of wrong because it will run the "correct" git-sh-setup *after* the one in /usr/bin, if there is one, which could be weird if you have multiple versions of git installed. But it works on my Linux and his msysgit, so it's obviously better than what we had before.