Lists filenames of migrations with numbers bigger than a given one.

def ListMigrationsToProcess(migrations_root,
                            current_migration_number
                           ):
  """Lists filenames of migrations with numbers bigger than a given one."""
  migrations = []
  for m in os.listdir(migrations_root):
    if (current_migration_number is None or
        _MigrationFilenameToInt(m) > current_migration_number):
      migrations.append(m)

  return sorted(migrations, key=_MigrationFilenameToInt)