submodule--helper: introduce new update-module-mode helper
This chews off a bit of the shell part of the update command in
git-submodule.sh. When writing the C code, keep in mind that the
submodule--helper part will go away eventually and we want to have
a C function that is able to determine the submodule update strategy,
it as a nicety, make determine_submodule_update_strategy accessible
for arbitrary repositories.
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>