[gecko]

That's pretty radically different. A closer analog might be hg-git, but the moment you say that you want the entire process to cleanly round-trip, you have to preserve all kinds of weird and corrupt data that the modern incarnations of the tools don't let you create (for good reason). There's also a lot of internal metadata, especially in Mercurial, where there are multiple valid ways to store a given concept, but only one will yield the right SHA for a given changeset. Not only did no existing tool try to handle these issues; they were all built in ways that would've made such preservation impossible. That's why we had to write our own from scratch.