range-diff: offer to dual-color the diffs

When showing what changed between old and new commits, we show a diff of
the patches. This diff is a diff between diffs, therefore there are
nested +/- signs, and it can be relatively hard to understand what is
going on.

With the --dual-color option, the preimage and the postimage are colored
like the diffs they are, and the *outer* +/- sign is inverted for
clarity.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/range-diff.c

@@ -20,9 +20,12 @@ 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;
 	struct option options[] = {
 		OPT_INTEGER(0, "creation-factor", &creation_factor,
 			    N_("Percentage by which creation is weighted")),
+		OPT_BOOL(0, "dual-color", &dual_color,
+			    N_("color both diff and diff-between-diffs")),
 		OPT_END()
 	};
 	int i, j, res = 0;
@@ -60,6 +63,11 @@ 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;
+		diffopt.flags.dual_color_diffed_diffs = 1;
+	}
+
 	if (argc == 2) {
 		if (!strstr(argv[0], ".."))
 			die(_("no .. in range: '%s'"), argv[0]);