merge: allow using --no-ff and --ff-only at the same time

1347483 (Teach 'git merge' and 'git pull' the option --ff-only,
2009-10-29) says this is not allowed, as they contradict each other.

However, --ff-only is about asserting the input of the merge, and
--no-ff is about instructing merge to always create a merge commit, i.e.
it makes sense to use these options together in some workflow.

Signed-off-by: Miklos Vajna <vmiklos@suse.cz>