diff: introduce diff.wsErrorHighlight option

With the preparatory steps, it has become trivial to teach the
system a new diff.wsErrorHighlight configuration that gives the
default value for --ws-error-highlight command line option.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-config.txt

@@ -182,3 +182,9 @@ diff.algorithm::
 	low-occurrence common elements".
 --
 +
+
+diff.wsErrorHighlight::
+	A comma separated list of `old`, `new`, `context`, that
+	specifies how whitespace errors on lines are highlighted
+	with `color.diff.whitespace`.  Can be overridden by the
+	command line option `--ws-error-highlight=<kind>`

Documentation/diff-options.txt

@@ -303,6 +303,8 @@ ifndef::git-format-patch[]
 	lines are highlighted.  E.g. `--ws-error-highlight=new,old`
 	highlights whitespace errors on both deleted and added lines.
 	`all` can be used as a short-hand for `old,new,context`.
+	The `diff.wsErrorHighlight` configuration variable can be
+	used to specify the default behaviour.
 
 endif::git-format-patch[]
 

diff.c

@@ -41,6 +41,7 @@ static int diff_stat_graph_width;
 static int diff_dirstat_permille_default = 30;
 static struct diff_options default_diff_options;
 static long diff_algorithm;
+static unsigned ws_error_highlight_default = WSEH_NEW;
 
 static char diff_colors[][COLOR_MAXLEN] = {
 	GIT_COLOR_RESET,
@@ -262,6 +263,14 @@ int git_diff_ui_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "diff.wserrorhighlight")) {
+		int val = parse_ws_error_highlight(value);
+		if (val < 0)
+			return -1;
+		ws_error_highlight_default = val;
+		return 0;
+	}
+
 	if (git_color_config(var, value, cb) < 0)
 		return -1;
 
@@ -3306,7 +3315,7 @@ void diff_setup(struct diff_options *options)
 	options->rename_limit = -1;
 	options->dirstat_permille = diff_dirstat_permille_default;
 	options->context = diff_context_default;
-	options->ws_error_highlight = WSEH_NEW;
+	options->ws_error_highlight = ws_error_highlight_default;
 	DIFF_OPT_SET(options, RENAME_EMPTY);
 
 	/* pathchange left =NULL by default */

t/t4015-diff-whitespace.sh

@@ -937,4 +937,39 @@ test_expect_success 'test --ws-error-highlight option' '
 
 '
 
+test_expect_success 'test diff.wsErrorHighlight config' '
+
+	git -c color.diff=always -c diff.wsErrorHighlight=default,old diff |
+	test_decode_color >current &&
+	test_cmp expect.default-old current &&
+
+	git -c color.diff=always -c diff.wsErrorHighlight=all diff |
+	test_decode_color >current &&
+	test_cmp expect.all current &&
+
+	git -c color.diff=always -c diff.wsErrorHighlight=none diff |
+	test_decode_color >current &&
+	test_cmp expect.none current
+
+'
+
+test_expect_success 'option overrides diff.wsErrorHighlight' '
+
+	git -c color.diff=always -c diff.wsErrorHighlight=none \
+		diff --ws-error-highlight=default,old |
+	test_decode_color >current &&
+	test_cmp expect.default-old current &&
+
+	git -c color.diff=always -c diff.wsErrorHighlight=default \
+		diff --ws-error-highlight=all |
+	test_decode_color >current &&
+	test_cmp expect.all current &&
+
+	git -c color.diff=always -c diff.wsErrorHighlight=all \
+		diff --ws-error-highlight=none |
+	test_decode_color >current &&
+	test_cmp expect.none current
+
+'
+
 test_done