submodule: correct remote name with fetch

submodule: correct remote name with fetch

The code fetches the submodules remote based on the superproject remote name
instead of the submodule remote name[1].

Instead of grabbing the default remote of the superproject repository, ask
the default remote of the submodule we are going to run 'git fetch' in.

1. https://lore.kernel.org/git/ZJR5SPDj4Wt_gmRO@pweza/

Signed-off-by: Daniel Black <daniel@mariadb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>