git add --all does not respect submodule.<name>.ignore

Giving a submodule "ignore=all" or "ignore=dirty" in .gitmodule successfully removes that module from `git status` queries. However, these same diffs are automatically added by git-add, eg `git add .` or `git add --all` adds the submodules that we want ignored. This seems inconsistent and confusing.

Workflow reason: We want to be able to have supers and subs checked-out, make changes to both, but only update the SHA-1 pointer from super to sub when explicitly forced to do so, eg `git add -f subName`. This workflow prevents continuous merge conflicts from clashing SHA-1 pointers while still allowing `git add --all`, allowing a sort of middle ground between submodules and an untracked library.

Teaching git-add about submodule.ignore and/or teaching .gitignore about submodules would be awesome.

Also experimented with `git update-index --assume-unchanged subName`,
but I believe that it does not get committed and besides also does not
seem to have a way to `git add -f`.
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Note: currently on git version 2.14.1, but looking through the
changelogs, did not see any changes since then that would enable this
workflow.

-Michael Scott-Nelson