pack-objects: remove bogus comment

pack-objects: remove bogus comment

The comment was introduced in b5d97e6 (pack-objects: run rev-list
equivalent internally. - 2006-09-04), stating that

git pack-objects [options] base-name <refs...>

is acceptable and refs should be passed into rev-list. But that's not
true. All arguments after base-name are ignored.

Remove the comment and reject this syntax (i.e. no more arguments after
base name)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>