A "git mv" that preserves history

This "issue" is more of a request for comments from anyone working on this repo that knows what they're talking about when it comes to git internals. In exchange for comments, I will happily turn this into a PR.

I "discovered" a way to move a directory that also preserves history, including dealing well with merges from code branched before the move: https://gist.github.com/dannyroberts/857b9f7e52acd6b83c89. I say "discovered" because I couldn't find instructions for this anywhere on the internet, but this is what is recommend for doing a "subtree merge" from another repository, and when I tested it, it also worked quite nicely for directories within the same repo.

I think it'd be really great to add to git extras—can any of you who know more than me think of why this might specifically be a bad idea?

Start by reviewing the linked gist and the issue's discussion of Git subtree merges and directory moves. No repository file or test is named, so first determine where this behavior could belong in git-extras and whether the proposed approach is safe. Done means resolving the request for expert comments and, if accepted, turning it into a PR.