Sorry, I was unclear.  There are two things getting confused here.

1 - the  abstract fs interface that js-git's fs-db consumes to perform it's fs operations (readFile, writeFile, readdir, readChunk).  The two existing modules, `git-chrome-db` and `git-node-db` provide this interface to form a complete mixin.  The `mem-db` mixin has nothing to do with this.

2 - the set of js-git mixins that forms the base of a full-featured js-git repo (loadAs, loadRaw, saveAs, saveRaw, readRef, updateRef, etc...)  This is basically a js-git storage backend.  Some backends implement more of this interface than others.  `mem-db`, `fs-db` (using either the chrome or node fs), `js-github/mixins/github-db`, `indexed-db`, etc are examples of this.