Feature request: --link

[nagya]

This option would change how non-converted attachments in all export formats, and database and other supporting files in a raw export (see #407) are copied: instead of clonefile(2), link(2) would be used to create hard links to the original files in the export. This would be useful when the user uses standard tooling (e.g. tar or rsync) to back up a directory hierarchy that contains both the original database and the export directory. Such tools recognize and handle hardlinks efficiently, but they don't detect when two files are clones of each other.