OPTION_CALLBACK is much simpler/safer to use, but parse_opt_cb does
not allow access to parse_opt_ctx_t, which sometimes is useful
(e.g. to obtain the prefix).

Extending parse_opt_cb to take parse_opt_cb could result in a lot of
changes. Instead let's just allow ll_callback to be used with
OPTION_CALLBACK. The user will have to be careful, not to change
anything in ctx, or return wrong result code. But that's the price for
ll_callback.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>