git 2.9 subtree split with rejoin

Hello,

It seems now with 2.9 you cannot use git subtree split —rejoin without —onto otherwise you get:
fatal: refusing to merge unrelated histories

I wish I could pass --allow-unrelated-histories but that doesn’t work.

Adding —onto makes the split operation much much longer. Could we make a bug fix which allow to pass the --allow-unrelated-histories flag to subtree split?

Thanks,
Sylvain.