[Borgbackup] extended prune

[Thomas Waldmann]

> In my understanding, the actual removal of chunks can be more efficient
> if performed in one go, is that correct?

Yes. borg prune does it in one go, but borg delete currently doesn't.

> Then it would be great to still have a prune option that takes a list,

No, let's keep borg prune as a "policy based deletion tool" (usually
time-based policy) and let's use borg delete if you already know what to
delete (e.g. by archive names).

borg prune is already confusing/complex enough, so we should not make it
even more complex.

> or alternatively a deletion that uses a separate garbage collection step.

Yes, I think changing do_delete / _delete_archives is the way to go.

If we can do it quick (with less commits) in do_prune, we also can do
that for deletion.

See archiver.py.

> Would one of these two be a reasonable extension to borg?

Yes, PRs welcome.

Something also missing for borg delete (and maybe a good idea to
implement FIRST) is a --dry-run option.

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