While there are a few differences in operating principles, probably 90% of the baseline knowledge required to use Git is the same as with Fossil (and Mercurial

While there are a few differences in operating principles, probably 90% of the baseline knowledge required to use Git is the same as with Fossil (and Mercurial for that matter). If you already know Git, you can learn Fossil in less than 30 minutes, and IME there is very little context switching cost if using multiple VCSs. In that sense, I don't believe Git has a knowledge lock in, because it is easy for anyone to switch or to simultaneously use available alternatives. It's all about the ecosystem, and really, Github.