Leverage alternate sources of data

[lourot]

In order to be able to crawl more data per day despite GitHub's API rate limit, we could look into making use of these sources, if not too expensive:

• https://cloud.google.com/bigquery/public-data/
• https://www.gharchive.org/
• git cloneing repos

[brillout]

About git cloneing repos: I've researched a bit in the past if it's somehow possible to git clone only the commit history without the actual changes but there doesn't seem to be such feature in git. Just to make sure I bountied https://stackoverflow.com/questions/37684028/how-to-clone-fetch-a-repo-getting-only-the-history

[brillout]

New interesting answer on SO: https://stackoverflow.com/a/52549508/270274

Not sure when GitHub will support it and if "GitHub support" means that all repoS on GitHub will be accessible with VFS for Git

[brillout]

@AurelienLourot I guess we should close in favor of the finer grained issues (21, 20, and 13)