Bugfix: `copy` failed to copy snapshots in rare cases

The `copy` command could in rare cases fail with the error message `SaveTree(...)
returned unexpected id ...`.  This has been fixed.

On Linux/BSDs, the error could be caused by backing up symlinks with non-UTF-8
target paths. Note that, due to limitations in the repository format, these are
not stored properly and should be avoided if possible.

https://github.com/restic/restic/issues/3267
https://github.com/restic/restic/pull/3310
