Option to ignore git submodules in recursion

[jannis-baum]

When recursively listing the contents of a small repository with one or more big submodules, it would be cool to be able to ignore the submodules similar to how gitignored files are ignored with --git-ignore. This would show only the submodules' top level directory and avoid unnecessary clutter in listing the original repository's contents.

Note: I tried grabbing the submodule paths in a sub-shell and using them with --ignore-glob but this doesn't work since --ignore-globs are only matched against file names and not directories as illustrated in #97 (comment).