Get the update rule that will be used for the submodule.
This value controls the behavior of the git submodule update
command.
There are four useful values documented with git_submodule_update_t
plus the GIT_SUBMODULE_UPDATE_RESET
which can be used to revert to
the on-disk setting.
The current git_submodule_update_t value that will be used for this submodule.