git-submodule.sh: remove duplicate call to set_rev_name

set_rev_name is a possiblly expensive operation. If a submodule has
changes in it, set_rev_name was called twice.

Move call to set_rev_name so it's only called once, no matter which
codepath is taken.

Signed-off-by: Fredrik Gustafsson <iveqy@iveqy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>