[Borgbackup] Change repository id

[Thomas Waldmann]

There are mainly 2 issues with cloning (making a 1:1 copy of) a repo:

- getting it "somehow working" - this needs changing of the repo id (see 
repo_dir/config to a unique new value) before you ever use the copy.

- getting it "cryptographically as secure as it can be" is maybe not 
easily possible as the data in repo1 and repo2 is encrypted with the 
same AES key and the same counter/IV values (AES CTR mode) and one can 
not assume that all future repo writes will be totally identical.

And if they aren't, you are breaking AES security due to counter/IV 
reuse (same key, same counter, different plaintext).

There are also some related infos about this in the borg docs/faq.

There is no code in borg yet to transfer an archive from one repo to 
another, but you could do extract / create for all archives, which might 
take quite a while depending on your archive count/size.

A hypothetical transfer command could be a bit better, but not much, as 
it would have to (at least) authenticate/decrypt/encrypt/authenticate 
all data. It could save writing all files to disk / reading them from 
disk again, though.

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