Ask HN: How would you setup a monorepo?

A monorepo is the idea that you use a single big repository for everything, instead of lots of small repositories (manyrepos). Manyrepos is currently en vogue. Google and Facebook are known companies that use a monorepo. Assume you are monorepo proponent. How would you implement it?

1. Use git (and hope monorepo supports comes as needed)

2. Use subversion (gives fine grained access permissions for example)

3. Use Mercurial (they seem to be ahead of git for big files and big repos, thanks to Facebook)

4. Use Bitkeeper since it is now Open Source and explicitly supports "very large repositories"?