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>

diff.c

@@ -4692,6 +4692,12 @@ static unsigned filter_bit_tst(char status, const struct diff_options *opt)
 	return opt->filter & filter_bit[(int) status];
 }
 
+unsigned diff_filter_bit(char status)
+{
+	prepare_filter_bits();
+	return filter_bit[(int) status];
+}
+
 static int diff_opt_diff_filter(const struct option *option,
 				const char *optarg, int unset)
 {