Diagram of the snapshot model of the Git data backend

[wlandau]

Prework

• [x] Read and agree to the Contributor Code of Conduct and contributing guidelines.
• [x] If there is already a relevant issue, whether open or closed, comment on the existing thread instead of posting a new issue.
• [n/a] For any problems you identify, post a minimal reproducible example like this one so the maintainer can troubleshoot. A reproducible example is:

Description

Would be helpful to include a visualization in the git.Rmd vignette.