Subtree split unsquashes everything

Hello! I was directed to ask here; I hope I am respecting your format.

I have a repo with a subtree. I squashed every merge with the subtree
remote to keep the history manageable. Now down the road after a bunch
of merges, I need to split my repo’s master branch into two new
branches and move the subtree from `/subdir/subdir/` to `/`.
(`/subdir/subdir/user/mycode/` contains code from my repo.)

I ran `git subtree split -P=subdir/subdir/ -b newbranch` and the
outcome seems to be perfect except that each squash merge has turned
into a full merge, bringing along all history from the other repo. Why
does it do this and how can I preserve my repo history, including only
squashes from the subtree remote repo like it is today?

I initially asked at http://stackoverflow.com/q/36957809

Thank you.