I've just tried running
bin-wrappers/git rebase -C1 @^
and I get
error: unknown switch `1' usage: git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] [<upstream>] [<branch>] or: git rebase [-i] [options] [--exec <cmd>] [--onto <newbase>] --root [<branch>] or: git rebase --continue | --abort | --skip | --edit-todo ...
bin-wrappers/git --version gives git version 2.19.1.856.g8858448bb4
Unfortunately I've not got time it investigate properly at the moment.
Best Wishes
Phillip