help.c: use 'git_config_string' to get 'help_default_format'.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -39,12 +39,8 @@ static void parse_help_format(const char *format)
 
 static int git_help_config(const char *var, const char *value)
 {
-	if (!strcmp(var, "help.format")) {
-		if (!value)
-			return config_error_nonbool(var);
-		help_default_format = xstrdup(value);
-		return 0;
-	}
+	if (!strcmp(var, "help.format"))
+		return git_config_string(&help_default_format, var, value);
 	return git_default_config(var, value);
 }