implement re-chunking for borg2 transfer?

[ThomasWaldmann]

Considering the CDC fingerprinting issues, we may choose to change how the chunker works, so it would cut chunks differently.

The problem with that is that this breaks deduplication with existing repository contents (chunks that were cut the old way), potentially doubling the repo storage needs.

Obviously, this is problematic as many people might run out of space.

When transferring repo contents to borg2, people will need to arrange new borg2 repos (and additional space for them) anyway, so that might be the perfect time to re-chunk with a new chunker.

So, borg2 transfer could maybe re-use some existing code we already have in borg recreate.

Note: even if we don't add a new chunker, this would be also useful if users want to change the parameters of the existing chunker.