submodule: move core cmd_update() logic to C

This patch completes the conversion past the flag parsing of
`submodule update` by introducing a helper subcommand called
`submodule--helper update`. The behaviour of `submodule update` should
remain the same after this patch.

Prior to this patch, `submodule update` was implemented by piping the
output of `update-clone` (which clones all missing submodules, then
prints relevant information for all submodules) into
`run-update-procedure` (which reads the information and updates the
submodule tree).

With `submodule--helper update`, we iterate over the submodules and
update the submodule tree in the same process. This reuses most of
existing code structure, except that `update_submodule()` now updates
the submodule tree (instead of printing submodule information to be
consumed by another process).

Recursing on a submodule is done by calling a subprocess that launches
`submodule--helper update`, with a modified `--recursive-prefix` and
`--prefix` parameter.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Shourya Shukla <periperidip@gmail.com>
Signed-off-by: Atharva Raykar <raykar.ath@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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