Set the update rule for the submodule.
The initial value comes from the ".git/config" setting of
submodule.$name.update
for this submodule (which is initialized from
the ".gitmodules" file). Using this function sets the update rule in
memory for the submodule. Call git_submodule_save()
to write out the
new update rule.
Calling this again with GIT_SUBMODULE_UPDATE_RESET or calling
git_submodule_reload()
will revert the rule to the on disk value.
old value for update