Use DIFF_DETECT_RENAME for detect_rename assignments

This field can have two values (2 for copy). Use this name instead for
clarity. Many places have already used this constant.

Note, the detect_rename assignments in merge-recursive.c remain
unchanged because it's actually a boolean there.

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

builtin/commit.c

@@ -1473,7 +1473,7 @@ static void print_summary(const char *prefix, const struct object_id *oid,
 	rev.show_root_diff = 1;
 	get_commit_format(format.buf, &rev);
 	rev.always_show_header = 0;
-	rev.diffopt.detect_rename = 1;
+	rev.diffopt.detect_rename = DIFF_DETECT_RENAME;
 	rev.diffopt.break_opt = 0;
 	diff_setup_done(&rev.diffopt);