transport-helper.c: refactor set_helper_option()

transport-helper.c: refactor set_helper_option()

For now we can handle two types, string and boolean, in
set_helper_option(). Later on we'll add string_list support, which does
not fit well. The new function strbuf_set_helper_option() can be reused
for a separate function that handles string-list.

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