submodule--helper: remove --prefix from "absorbgitdirs"

submodule--helper: remove --prefix from "absorbgitdirs"

Let's pass the "-C <prefix>" option instead to "absorbgitdirs" from
its only caller.

When it was added in f6f85861400 (submodule: add absorb-git-dir
function, 2016-12-12) there were other "submodule--helper" subcommands
that were invoked with "-C <prefix>", so we could have done this all
along.

Suggested-by: Glen Choo <chooglen@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>