Add a naive git bare cooker

It can cook directories (by adding a synthetic revision pointing to it)
and revisions.

Current limitations:

• It does not deduplicate directories and files at all, and queries all objects one by one.
• No support for missing/absent contents
• No support for missing submodules

Tests reuse existing tests of the DirectoryCooker and
RevisionGitfastCooker using parametrized pytest fixtures.