Rework files manipulation

[durierem]

Addition to the master tree

TODO

When a file is in the master tree

tldr: differentiate removing from unlinking

• when there is no corresponding file in the file system
  • link => create a corresponding symlink
  • remove => remove the file from the master tree
• when there is a corresponding file in the file system
  • when the file is a symlink
    • untrack => replace the symlink by a copy a the file in the master tree
    • unlink => remove the symlink
    • remove => remove the file from the master tree with a warning that the corresponding symlink will be obsolete
  • when the file is... a file
    • remove => remove the file from the master tree
    • `