Users may be confused when they run the perl script directly.
A good way to detect this is to check the number of parameters used to call the
script, which is never different from 2 in a normal use.
Display a proper error message to avoid any confusion.
Signed-off-by: Célestin Matte <celestin.matte@ensimag.fr> Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr> Signed-off-by: Junio C Hamano <gitster@pobox.com>