fetchcode take long time to download

sometimes we just need a small part of the repository like https://github.com/github/advisory-database
we just need to `github-reviewed` part `57.2 MB`. but using `fetch_via_vcs` we download `2.00 GiB`
the problem gets worth when we start rerunning the importer and debugging it,
at least we can have something like a cache for saving the repo so we can use it again.
https://github.com/github/advisory-database/tree/main/advisories/github-reviewed

nexB/vulnerablecode/issues/902