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>

contrib/completion/git-completion.bash

@@ -1981,7 +1981,7 @@ _git_range_diff ()
 	case "$cur" in
 	--*)
 		__gitcomp "
-			--creation-factor= --dual-color
+			--creation-factor= --no-dual-color
 			$__git_diff_common_options
 		"
 		return