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>

diff.c

@@ -218,7 +218,7 @@ static int parse_ws_error_highlight(const char *arg)
  */
 void init_diff_ui_defaults(void)
 {
-	diff_detect_rename_default = 1;
+	diff_detect_rename_default = DIFF_DETECT_RENAME;
 }
 
 int git_diff_heuristic_config(const char *var, const char *value, void *cb)