Implement rdiff-backup support

[leegarrett]

https://rdiff-backup.net/ is another common backup tool. Implementing the importer might be a bit easier due to the fact that it's also written in python.

I've written a shell script that does it, however it's quite cumbersome as it requires the full extraction of a snapshot for borg to back it up, and rdiff-backup isn't very smart on restore (it restores a file even if it's unchanged).

[ThomasWaldmann]

I'ld guess for a simple implementation that would be also the way to go with borg-import importing such backups.

With a lot of effort one could theoretically maybe avoid that full extractions get written to disk, but I guess that would be too much work for just single-time use (per user). It's not just the file names and content, we also would need to care for all the other metadata.