Added word-diff=plain colorizing support

Added ADD and DEL colors for {+added+} and [-removed-] blocks of git word-diff=plain

Since git does not escape the delimiters and splits them by line, this code would sometimes end up with an incorrect highlighting.

The solution scans DEFAULT lines of the diff chunk and and adds colored cells. Since git (unlike wdiff) does not use multi-line word-diff, it works.

src/diff.c

@@ -210,6 +210,79 @@ diff_common_add_diff_stat(struct view *view, const char *text, size_t offset)
 	return NULL;
 }
 
+static bool
+diff_common_read_diff_wdiff_group(struct diff_stat_context *context)
+{
+	const char *sep_add = strstr(context->text, "{+");
+	const char *sep_del = strstr(context->text, "[-");
+
+	if (sep_add == NULL && sep_del == NULL)
+		return false;
+
+	const char * sep;
+	enum line_type next_type;
+	const char* end_delimiter;
+
+	if (sep_del == NULL || (sep_add != NULL && sep_add < sep_del)) {
+		sep = sep_add;
+		next_type = LINE_DIFF_ADD;
+		end_delimiter = "+}";
+	} else {
+		sep = sep_del;
+		next_type = LINE_DIFF_DEL;
+		end_delimiter = "-]";
+	}
+
+	diff_common_add_cell(context, sep - context->text, false);
+
+	context->type = next_type;
+	context->text = sep;
+
+	// workaround for a single }/] change
+	const char* end_sep = strstr(context->text + sizeof("{+") - 1, end_delimiter);
+
+	size_t len;
+	if (end_sep == NULL) {
+		// diff is not terminated
+		len = strlen(context->text);
+	} else {
+		// separators are included in the add/del highlight
+		len = end_sep - context->text + sizeof("+}") - 1;
+	}
+
+	diff_common_add_cell(context, len, false);
+
+	if (end_sep == NULL) {
+		context->text += len;
+	} else {
+		context->text = end_sep + sizeof("+}") - 1;
+	}
+	context->type = LINE_DEFAULT;
+
+	return true;
+}
+
+static struct line *
+diff_common_read_diff_wdiff(struct view *view, const char *text)
+{
+	struct diff_stat_context context = { text, LINE_DEFAULT };
+
+
+	/* Detect remaining part of a word diff line:
+	 *
+	 *	added {+new +} text
+	 *	removed[- something -] from the file
+	 *	replaced [-some-]{+same+} text
+	 *	there could be [-one-] diff part{+s+} in the {+any +} line
+	 */
+
+	while (diff_common_read_diff_wdiff_group(&context));
+
+	diff_common_add_cell(&context, strlen(context.text), false);
+
+	return diff_common_add_line(view, text, LINE_DEFAULT, &context);
+}
+
 static bool
 diff_common_highlight(struct view *view, const char *text, enum line_type type)
 {
@@ -270,7 +343,10 @@ diff_common_read(struct view *view, const char *data, struct diff_state *state)
 		return line != NULL;
 	}
 
-	if (type == LINE_DIFF_HEADER) {
+	if (state->reading_diff_chunk && type == LINE_DEFAULT) {
+		if (diff_common_read_diff_wdiff(view, data))
+			return true;
+	} else if (type == LINE_DIFF_HEADER) {
 		state->after_diff = true;
 		state->reading_diff_chunk = false;