`git-cp` does not work if `merge.ff = only`

[halostatue]

$ git cp need_git_commit x
Coping need_git_commit into x
[master a9e4e94279bf] Keep need_git_commit
 1 file changed, 0 insertions(+), 0 deletions(-)
 rename need_git_commit => need_git_commit-move-to-x (100%)
HEAD is now at cb29c62c65bf readme: Add secondary location of screencast (#1065)
[master 3d021c144faf] Copy need_git_commit into x
 1 file changed, 0 insertions(+), 0 deletions(-)
 rename need_git_commit => x (100%)
HEAD is now at cb29c62c65bf readme: Add secondary location of screencast (#1065)
hint: Diverging branches can't be fast-forwarded, you need to either:
hint:
hint: 	git merge --no-ff
hint:
hint: or:
hint:
hint: 	git rebase
hint:
hint: Disable this message with "git config advice.diverging false"
fatal: Not possible to fast-forward, aborting.

This can be fixed by adding --no-ff to the git merge at bin/git-cp:60.