honor hidden files when using the pijul add subcommand

Dependencies

• [2] 3J6IK4W2 Explicitly adding .pijul is now forbidden
• [3] IUH7IMWE Contributor license agreements
• [*] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• edit in libpijul/src/working_copy/filesystem.rs at line 161
  [2.345]

  [2.345]
  + .hidden(false)