pull: display default warning only when non-ff

pull: display default warning only when non-ff

There's no need to display the annoying warning on every pull... only
the ones that are not fast-forward.

The current warning tests still pass, but not because of the arguments
or the configuration, but because they are all fast-forward.

We need to test non-fast-forward situations now.

Suggestions-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>