This introduces the functionality of submodule update in
'git_submodule_do_update'. The existing 'git_submodule_update' function is
renamed to 'git_submodule_update_strategy'. The 'git_submodule_update'
function now refers to functionality similar to `git submodule update`,
while `git_submodule_update_strategy` is used to get the configured value
of submodule.<name>.update.
