Add missing DB rollback functionality

lib/private/db.php

@@ -204,6 +204,13 @@ class OC_DB {
 		return \OC::$server->getDatabaseConnection()->commit();
 	}
 
+	/**
+	 * Rollback the database changes done during a transaction that is in progress
+	 */
+	public static function rollback() {
+		return \OC::$server->getDatabaseConnection()->rollback();
+	}
+
 	/**
 	 * saves database schema to xml file
 	 * @param string $file name of file

lib/public/db.php

@@ -96,6 +96,13 @@ class DB {
 		\OC_DB::commit();
 	}
 
+	/**
+	 * Rollback the database changes done during a transaction that is in progress
+	 */
+	public static function rollback() {
+		\OC_DB::rollback();
+	}
+
 	/**
 	 * Check if a result is an error, works with Doctrine
 	 * @param mixed $result