Really useful info, I know of at least one open source and one proprietary software project both of which could put this to great use. It it certainly less documented than the submodule workflow. And I find 50% of the joy of git come from switching to it, the rest from learning to use it to do cool stuff. The first time I used git bisect it made my day, no my week!
I won't say any of this is hot, new information. This is my attempt to document the use of the subtree merge strategy/workflow in a way that would have helped me in figuring it out. While most of the information was out there, my best understanding came from working through multiple sources. If anyone sees where I have something wrong, I would love to know so I can correct it. I do NOT want to be propagating bad information. There is a real basic walk through for folks who are not deeply experienced with Git, and also a link to more of a reference for those who are more fluent and need less of the narrative/screenshots. As always, feedback is appreciated.