whatchanged: require --i-still-use-this

whatchanged: require --i-still-use-this

The documentation of "git whatchanged" is pretty explicit that the
command was retained for historical reasons to help those whose fingers
cannot be retrained.  Let's see if they still are finding it hard to
type "git log --raw" instead of "git whatchanged" by marking the
command as "nominated for removal", and require "--i-still-use-this"
on the command line.  Adjust the tests so that the option is passed
when we invoke the command.  In addition, we test that the command
fails when "--i-still-use-this" is not given.

Signed-off-by: Junio C Hamano <gitster@pobox.com>