submodule update: add regression test with old style setups

As f178c13fda (Revert "Merge branch 'sb/submodule-core-worktree'",
2018-09-07) was produced shortly before a release, nobody asked for
a regression test to be included. Add a regression test that makes sure
that the invocation of `git submodule update` on old setups doesn't
produce errors as pointed out in f178c13fda.

The place to add such a regression test may look odd in t7412, but
that is the best place as there we setup old style submodule setups
explicitly.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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