When `merge.ff` is set to `only` in .gitconfig, `git subtree pull` will
fail with error `fatal: Not possible to fast-forward, aborting.`, but
the command does want to make merges in these places. Add `--no-ff`
argument to `git merge` to enforce this behaviour.
Signed-off-by: Thomas Koutcher <thomas.koutcher@online.fr> Reviewed-by: Johannes Altmanninger <aclopte@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>