add passphrase debugging option?

[ThomasWaldmann]

Sometimes people have issues when trying to access a borg repo from a new or newly installed host with borg telling them that the passphrase is incorrect.

Besides the obvious root cause of the passphrase really being incorrect, there are also:

• keyboard issues (kbd mapping, deadkey vs nodeadkey with accents in passphrases, ...)
• differences in unicode normalisation between platforms
• troubles correctly setting BORG_PASSPHRASE, BORG_PASSCOMMAND, ... env vars

borg init (borg 1.x) and borg repo-create (borg 2) show the utf-8 hex bytes sequence of the encoded passphrase IF the passphrase is non-ascii.

We could:

• init: display the hex bytes in any case (the deadkey issue can also happen within ascii, e.g. if the original passphrase has ^a and the wrong one happens to have â).
• add an option to display the password and/or hex bytes when the password is USED and is determined to be incorrect.
• show which env vars were used