previously, the assumption was made that the database and filesystem
would not desync, and that an upload could safely be substituted with
a symlink to an existing copy on-disk, assuming said copy still
existed on-disk at all
this is fine if copyparty is the only software that makes changes to
the filesystem, but that is a shitty assumption to make in hindsight
add `--safe-dedup` which takes a "safety level", and by default (50)
it will no longer blindly expect that the filesystem has not been
altered through other means; the file contents will now be hashed
and compared to the database
deduplication can be much slower as a result, but definitely worth it
as this avoids some potentially very unpleasant surprises
the previous behavior can be restored with `--safe-dedup 1`
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