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>

Documentation/git-range-diff.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git range-diff' [--color=[<when>]] [--no-color] [<diff-options>]
-	[--dual-color] [--creation-factor=<factor>]
+	[--no-dual-color] [--creation-factor=<factor>]
 	( <range1> <range2> | <rev1>...<rev2> | <base> <rev1> <rev2> )
 DESCRIPTION
@@ -31,11 +31,14 @@ all of their ancestors have been shown.
 OPTIONS
 -------
---dual-color::
-	When the commit diffs differ, recreate the original diffs'
-	coloring, and add outer -/+ diff markers with the *background*
-	being red/green to make it easier to see e.g. when there was a
-	change in what exact lines were added.
+--no-dual-color::
+	When the commit diffs differ, `git range-diff` recreates the
+	original diffs' coloring, and adds outer -/+ diff markers with
+	the *background* being red/green to make it easier to see e.g.
+	when there was a change in what exact lines were added. This is
+	known to `range-diff` as "dual coloring". Use `--no-dual-color`
+	to revert to color all lines according to the outer diff markers
+	(and completely ignore the inner diff when it comes to color).
 --creation-factor=<percent>::
 	Set the creation/deletion cost fudge factor to `<percent>`.
@@ -118,15 +121,16 @@ line (with a perfect match) is yellow like the commit header of `git
 show`'s output, and the third line colors the old commit red, the new
 one green and the rest like `git show`'s commit header.
-The color-coded diff is actually a bit hard to read, though, as it
-colors the entire lines red or green. The line that added "What is
-unexpected" in the old commit, for example, is completely red, even if
-the intent of the old commit was to add something.
-To help with that, use the `--dual-color` mode. In this mode, the diff
-of diffs will retain the original diff colors, and prefix the lines with
--/+ markers that have their *background* red or green, to make it more
-obvious that they describe how the diff itself changed.
+A naive color-coded diff of diffs is actually a bit hard to read,
+though, as it colors the entire lines red or green. The line that added
+"What is unexpected" in the old commit, for example, is completely red,
+even if the intent of the old commit was to add something.
+To help with that, `range` uses the `--dual-color` mode by default. In
+this mode, the diff of diffs will retain the original diff colors, and
+prefix the lines with -/+ markers that have their *background* red or
+green, to make it more obvious that they describe how the diff itself
+changed.
 Algorithm

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;
 	}

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