> Currently at least the `srfi clone` command is affected.

What does `srfi clone` do and why does it need to know the default branch?

I'm almost sure there's a way to get the default branch of a Git
repository but I'm not finding it now...

Depending on purpose, since `git clone` clones the default branch unless
otherwise instructed, you can `git branch --show-current` to get the
current (thus default) branch.