reset: reject --no-(mixed|soft|hard|merge|keep) option

reset: reject --no-(mixed|soft|hard|merge|keep) option

"git reset --no-mixed" behaved exactly like "git reset --mixed",
which was nonsense.

If there were only two kinds, e.g. "mixed" vs "separate", it might
have made sense to make "git reset --no-mixed" behave identically to
"git reset --separate" and vice-versa, but because we have many
types of reset, let's just forbid "--no-mixed" and negated form of
other types.

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