Use multiple --link-dest arguments when possible

[TDurrr1]

Checklist

• I'm asking for a feature to be changed or added
• I've searched the bugtracker for similar issues, including closed ones.
• I realise that rsnapshot is stable software and most feature requests will be rejected

My suggestion is ...

A configuration option should be added (or the functionality should be enabled by default) to have rsnapshot generate multiple --link-dest arguments for rsync, up to rsync's limit of twenty.

Why I want it

(Let me know if I'm misunderstanding anything.)

Currently, rsnapshot only generates a --link-dest to the single most recent snapshot. This doesn't necessarily utilize that option to its fullest. For example, with daily backups, a file which is alternately moved into and out of the source directory would never be hardlinked. Using multiple --link-dests would enable files like this to still be properly hardlinked and reduce backup sizes. More generally, it would also simply give rsnapshot more chances to hardlink files which may move around but eventually return to previous locations.

A configuration option would make this easy to modify as well, something along the lines of link_dest_range daily to limit to a retain definition or link_dest_count 20 to limit to the max rsync can handle.

Why should this be in rsnapshot and not implemented using some external tool?

Admittedly, I haven't searched for many external tools. Since rsnapshot is generating the --link-dest argument in the first place, it seems apt and able to just generate more of them in the same place within this tool. If there's already a way to do this, that would be fantastic!