Get the ignore rule for the submodule.

There are four ignore values:

• GIT_SUBMODULE_IGNORE_NONE will consider any change to the contents of the submodule from a clean checkout to be dirty, including the addition of untracked files. This is the default if unspecified.
  • GIT_SUBMODULE_IGNORE_UNTRACKED examines the contents of the working tree (i.e. call git_status_foreach() on the submodule) but UNTRACKED files will not count as making the submodule dirty.
  • GIT_SUBMODULE_IGNORE_DIRTY means to only check if the HEAD of the submodule has moved for status. This is fast since it does not need to scan the working tree of the submodule at all.
  • GIT_SUBMODULE_IGNORE_ALL means not to open the submodule repo. The working directory will be consider clean so long as there is a checked out version present.