Improve patch to use git --exec-path: add to PATH instead.

If you (like me) are using a modified git straight out of its source
directory (ie. without installing), then --exec-path isn't actually correct.
Add it to the PATH instead, so if it is correct, it'll work, but if it's
not, we fall back to the previous behaviour.

git-subtree.sh

@@ -27,7 +27,8 @@ rejoin        merge the new branch back into HEAD
 squash        merge subtree changes as a single commit
 "
 eval $(echo "$OPTS_SPEC" | git rev-parse --parseopt -- "$@" || echo exit $?)
-. $(git --exec-path)/git-sh-setup
+PATH=$(git --exec-path):$PATH
+. git-sh-setup
 require_work_tree
 
 quiet=