Can we make git clone --recurse-submodules --shallow-submodules work for commits that are not on tags or branches

[Ciro Santilli <ciro.santilli@gmail.com>]

I have a git repo with large submodules, notably the Linux kernel, and
shallow clone saves a lot of time.

However, QEMU is also a submodule, which I don't control, and QEMU has
submodules which don't point to commits that are not on any tag or
branch, so:

 git clone --recurse-submodules --shallow-submodules git://git.qemu.org/qemu.git

currently fails with:

error: Server does not allow request for unadvertised object
22ead3e0bfdb87516656453336160e0a37b066bf
Fetched in submodule path 'capstone', but it did not contain
22ead3e0bfdb87516656453336160e0a37b066bf. Direct fetching of that
commit failed.

on git 2.16.1.

Furthermore, I reproduce this locally with direct filesystem clones:
https://github.com/cirosantilli/test-git-web-interface/blob/15335d3002a3e64fc5756b69fb832a733aa63fb9/shallow-submodule.sh#L158
and on GitHub, so I'm guessing it is not just the settings for a
specific server?

Would it be possible to make that work, or are there fundamental
reasons why it is not possible?

Here is my use case repo, at the point of the ugly workaround I'm
having to do: https://github.com/cirosantilli/linux-kernel-module-cheat/blob/a14c95346cfd9d2e7b2e475b0981aa71d819c20b/configure#L23

Some more context:
https://stackoverflow.com/questions/2144406/git-shallow-submodules/47374702#47374702

This would make some embedded people happy.