Add support for openssh signatures

Git is about to gain ability to support openssh signatures, so introduce
this as a supported signature format for patatt.

To enable:

    [patatt]
        signingKey = openssh:~/.ssh/your-key-id.pub

Since openssh supports a number of crypto/hashing algorithms, this is
not algorithm-specific just as openpgp sigs are.

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
5 files changed