submodule--helper: convert "{update,clone}" to their own "--super-prefix"

submodule--helper: convert "{update,clone}" to their own "--super-prefix"

As with a preceding commit to convert "absorbgitdirs", we can convert
"submodule--helper status" to use its own "--super-prefix", instead of
relying on the global "--super-prefix" argument to "git".

We need to convert both of these away from the global "--super-prefix"
at the same time, because "update" will call "clone", but "clone"
itself didn't make use of the global "--super-prefix" for displaying
paths. It was only on the list of sub-commands that accepted it
because "update"'s use of it would set it in its environment.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>