Set the ignore rule for the submodule.

This sets the ignore rule in memory for the submodule. This will be used for any following actions (such as git_submodule_status()) while the submodule is in memory. You should call git_submodule_save() if you want to persist the new ignore role.

Calling this again with GIT_SUBMODULE_IGNORE_DEFAULT or calling git_submodule_reload() will revert the rule to the value that was in the original config.