submodule--helper: rename "absorb-git-dirs" to "absorbgitdirs"

submodule--helper: rename "absorb-git-dirs" to "absorbgitdirs"

Rename the "absorb-git-dirs" subcommand to "absorbgitdirs", which is
what the "git submodule" command itself has called it since the
subcommand was implemented in f6f85861400 (submodule: add
absorb-git-dir function, 2016-12-12).

Having these two be different will make it more tedious to dispatch to
eventually dispatch "git submodule--helper" directly, as we'd need to
retain this name mapping. So let's get rid of this needless
inconsistency.

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