range-diff: make --dual-color the default mode

After using this command extensively for the last two months, this
developer came to the conclusion that even if the dual color mode still
leaves a lot of room for confusion about what was actually changed, the
non-dual color mode is substantially worse in that regard.

Therefore, we really want to make the dual color mode the default.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/range-diff.c b/builtin/range-diff.c
index 5a9ad82..f52d45d 100644
--- a/builtin/range-diff.c
+++ b/builtin/range-diff.c
@@ -20,11 +20,11 @@ int cmd_range_diff(int argc, const char **argv, const char *prefix)
 {
 	int creation_factor = 60;
 	struct diff_options diffopt = { NULL };
-	int dual_color = 0;
+	int simple_color = -1;
 	struct option options[] = {
 		OPT_INTEGER(0, "creation-factor", &creation_factor,
 			    N_("Percentage by which creation is weighted")),
-		OPT_BOOL(0, "dual-color", &dual_color,
+		OPT_BOOL(0, "no-dual-color", &simple_color,
 			    N_("color both diff and diff-between-diffs")),
 		OPT_END()
 	};
@@ -63,8 +63,10 @@ int cmd_range_diff(int argc, const char **argv, const char *prefix)
 			     options + ARRAY_SIZE(options) - 1, /* OPT_END */
 			     builtin_range_diff_usage, 0);
 
-	if (dual_color) {
-		diffopt.use_color = 1;
+	if (simple_color < 1) {
+		if (!simple_color)
+			/* force color when --dual-color was used */
+			diffopt.use_color = 1;
 		diffopt.flags.dual_color_diffed_diffs = 1;
 	}