Fix git signing on ledger devices

[adambor]

This PR fixes git signing on Ledger devices by using the 0x06 (ins_sign_generic_hash) instruction instead of 0x04 (ins_sign_ssh_blob), related to LedgerHQ/app-ssh-agent#47

Tested working on linux, with Ledger Nano X and SSH/PGP ledger app version 0.0.8.

[adambor]

Okay, so just realized it's not really a bug but more so a ledger app expecting a certain format of the message to be signed (the one used when connecting over SSH). However trying to use the agent for e.g. signing git commits uses a different message format and that then breaks the implementation. So really what should be done in the agent:

• check if the message is conforming to the format expected by the app (duplicate the ledger app parsing logic in the agent):
  • if yes then use the ins_sign_ssh_blob(), type 0x04, which also nicely displays the SSH username
  • if no then use the ins_sign_generic_hash(), type 0x06, which displays just Sign PGP on the ledger device.

Will work on this and then push to this PR.