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>

builtin/am.c

@@ -1515,11 +1515,11 @@ static int fall_back_threeway(const struct am_state *state, const char *index_pa
 		 * review them with extra care to spot mismerges.
 		 */
 		struct rev_info rev_info;
-		const char *diff_filter_str = "--diff-filter=AM";
 
 		repo_init_revisions(the_repository, &rev_info, NULL);
 		rev_info.diffopt.output_format = DIFF_FORMAT_NAME_STATUS;
-		diff_opt_parse(&rev_info.diffopt, &diff_filter_str, 1, rev_info.prefix);
+		rev_info.diffopt.filter |= diff_filter_bit('A');
+		rev_info.diffopt.filter |= diff_filter_bit('M');
 		add_pending_oid(&rev_info, "HEAD", &our_tree, 0);
 		diff_setup_done(&rev_info.diffopt);
 		run_diff_index(&rev_info, 1);

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)
 {

diff.h

@@ -233,6 +233,8 @@ struct diff_options {
 	struct option *parseopts;
 };
 
+unsigned diff_filter_bit(char status);
+
 void diff_emit_submodule_del(struct diff_options *o, const char *line);
 void diff_emit_submodule_add(struct diff_options *o, const char *line);
 void diff_emit_submodule_untracked(struct diff_options *o, const char *path);