am: avoid diff_opt_parse()

am: avoid diff_opt_parse()

diff_opt_parse() is a heavy hammer to just set diff filter. But it's
the only way because of the diff_status_letters[] mapping. Add a new
API to set diff filter and use it in git-am. diff_opt_parse()'s only
remaining call site in revision.c will be gone soon and having it here
just because of git-am does not make sense.

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