fix: drop symlinks in immer subtree

I've been playing around recently with the `github-merge.py` script recently; and while I've been able to create merges with it (see 73c90c8 and 10ddf62)
it complains / refuses to trivially work with symlinks. I haven't been able to figure out exactly why it doesn't want symlinks, but I think it's related to the sha512sum hash that gets created.

I'm not sure if we'll start using `github-merge.py` more or not; but I guess we should "fix" this anyhow in order to learn more how these work. It may be useful to move to merging commits locally and not relying on github being a good actor? 🤷