svn primer translation to git

[Ed Maste]

We currently have a FreeBSD Subversion primer in the committer's guide:
https://www.freebsd.org/doc/en_US.ISO8859-1/articles/committers-guide/subversion-primer.html

I've started a translation of this into a Git primer, at:
https://hackmd.io/ML5TSl8mQ5-27B5eqDf7YA?view

I'm particularly interested in feedback on how much git background /
theory of operation we want to include here (vs referring to external
documentation).