pull: abort if --ff-only is given and fast-forwarding is impossible

pull: abort if --ff-only is given and fast-forwarding is impossible

The warning about pulling without specifying how to reconcile divergent
branches says that after setting pull.rebase to true, --ff-only can
still be passed on the command line to require a fast-forward. Make that
actually work.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
[en: updated tests; note 3 fixes and 1 new failure]
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>