92c62a3 (Porcelain scripts: Rewrite cryptic "needs update" error
message, 2010-10-19) refactored git's own checking to a function in
git-sh-setup. This is a very useful thing for script writers, so
document it.
Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>