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

[Michael Scott-Nelson <mscottnelson@gmail.com>]

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`.
---
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