diff-index does not consider a removed submodule to be staged with --ignore-submodules

After staging the removal of a submodule, diff-index does not consider this when "--ignore-submodules" is being used:

    # In a repository with submodule "sm":
    % git rm --cached sm
    % git diff-index --cached --quiet --ignore-submodules HEAD
    % echo $?
    0
    % git status
    On branch master
    Changes to be committed:
      (use "git reset HEAD <file>..." to unstage)

        deleted:    sm

"git status --ignore-submodules" behaves the same.

This command is used by zsh's vcs_info module (in Functions/VCS_Info/Backends/VCS_INFO_get_data_git):

    if (( querystaged )) ; then
        if ${vcs_comm[cmd]} rev-parse --quiet --verify HEAD &> /dev/null ; then
            ${vcs_comm[cmd]} diff-index --cached --quiet --ignore-submodules HEAD 2> /dev/null
            (( $? && $? != 128 )) && gitstaged=1

Is this a bug/oversight in Git or by design? Is there a better way to detect if there are any staged changes?

Regards, Daniel.