"git submodule update --depth 1" may fail with old Git

I'm working on MDEV-20554 (updating bundled Mroonga) with "git submodules" approach.

We copy the Mroonga source into storage/mroonga for now. I'll remove storage/mroonga and use "git submodules" instead for MDEV-20554.

We use "git submodule --init --recursive --depth 1" to update submodules in CI.
But old Git (< 2.8) may fail it when a submodule refers an old commit hash.

e.g.: https://buildbot.askmonty.org/buildbot/builders/kvm-tarbake-jaunty-x86/builds/46427/steps/compile/logs/stdio

fatal: reference is not a tree: 2ceda83fd7880d00eaf1a84a835c466c0167d46c

See also a commit for Git 2.8: https://github.com/git/git/commit/fb43e31f2b43076e7a30c9cd00d024