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.

git-subtree.sh

@@ -30,10 +30,8 @@ squash        merge subtree changes as a single commit
 "
 eval $(echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?)
 
-OPATH=$PATH
-PATH=$(git --exec-path):$PATH
+PATH=$PATH:$(git --exec-path)
 . git-sh-setup
-PATH=$OPATH  # apparently needed for some versions of msysgit
 
 require_work_tree