Set the ignore rule for the submodule.

This sets the in-memory ignore rule for the submodule which will control the behavior of git_submodule_status().

To make changes persistent, call git_submodule_save() to write the value to disk (in the ".gitmodules" and ".git/config" files).

Call with GIT_SUBMODULE_IGNORE_RESET or call git_submodule_reload() to revert the in-memory rule to the value that is on disk.