git-sh-setup.sh: use dashdash with basename call

Calling basename on a argument that starts with a dash, like a login
shell, will result in an error. Add '--' before the argument so that
the argument is interpreted properly.

Signed-off-by: Dan Wyand <danwyand@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>