Git repositories using submodules do not work

[andreabedini]

input-output-hk/hydra is in this unfortunate situation.

We fetch a tarball for the repo using GitHub API. Like

❯ curl -L \
  -H "Accept: application/vnd.github+json" \
  -H "X-GitHub-Api-Version: 2022-11-28" \
  https://github.com/input-output-hk/hydra/tarball/9f1027e0fdff6765f5233f19c4639fdaa3558bfa -o tmp.tgz

but that tarball does not contain the git submodules. It does include .gitmodules

❯ cat .gitmodules
[submodule "testnet/cardano-configurations"]
        path = hydra-cluster/config/cardano-configurations
        url = https://github.com/input-output-hk/cardano-configurations.git
[submodule "hydra-node/json-schemas/cardanonical"]
        path = hydra-node/json-schemas/cardanonical
        url = https://github.com/CardanoSolutions/cardanonical

but AFAIU the information about what commit they are supposed to be checkout out at is stored in the repository's .git folder, and therefore not part of the tarball.

It seems possible to obtain that information with a single api query

❯ curl -L \
  -H "Accept: application/vnd.github+json" \
  -H "X-GitHub-Api-Version: 2022-11-28" \
  https://api.github.com/repos/input-output-hk/hydra/contents/hydra-cluster/config/cardano-configurations\?ref=9f1027e0fdff6765f5233f19c4639fdaa3558bfa
{
  "name": "cardano-configurations",
  "path": "hydra-cluster/config/cardano-configurations",
  "sha": "f414e7b59a0f100d340b7368bc474e023c26c9f5",
  "size": 0,
  "url": "https://api.github.com/repos/input-output-hk/hydra/contents/hydra-cluster/config/cardano-configurations?ref=9f1027e0fdff6765f5233f19c4639fdaa3558bfa",
  "html_url": "https://github.com/input-output-hk/cardano-configurations/tree/f414e7b59a0f100d340b7368bc474e023c26c9f5",
  "git_url": "https://api.github.com/repos/input-output-hk/cardano-configurations/git/trees/f414e7b59a0f100d340b7368bc474e023c26c9f5",
  "download_url": null,
  "type": "submodule",
  "submodule_git_url": "https://github.com/input-output-hk/cardano-configurations.git",
  "_links": {
    "self": "https://api.github.com/repos/input-output-hk/hydra/contents/hydra-cluster/config/cardano-configurations?ref=9f1027e0fdff6765f5233f19c4639fdaa3558bfa",
    "git": "https://api.github.com/repos/input-output-hk/cardano-configurations/git/trees/f414e7b59a0f100d340b7368bc474e023c26c9f5",
    "html": "https://github.com/input-output-hk/cardano-configurations/tree/f414e7b59a0f100d340b7368bc474e023c26c9f5"
  }
}

but #80 is also an alternative.