def write_sync_control_file(branch_name)
      latest_sha = rev_parse(branch_name)
      filename = sync_control_filename(branch_name)
      logger.debug { "Writing sync control file, #{filename}, with #{latest_sha}" }
      File.open(filename, 'w') { |f| f.puts latest_sha }
    end
def write_sync_control_file(branch_name)
      latest_sha = rev_parse(branch_name)
      filename = sync_control_filename(branch_name)
      logger.debug { "Writing sync control file, #{filename}, with #{latest_sha}" }
      File.open(filename, 'w') { |f| f.puts latest_sha }
    end
Writes the current SHA-1 for the tip of the branch to the "sync control file"

 @return [void]
 @see GitLib#read_sync_control_file