expand key ID references in commit messages

[ktdreyer]

https://evil32.com/ explains why we should reduce the use of 32-bit key IDs.

git-crypt writes commit messages with short key IDs:

// given a key fingerprint, return the last 8 nibbles
std::string gpg_shorten_fingerprint (const std::string& fingerprint)
{
    return fingerprint.size() == 40 ? fingerprint.substr(32) : fingerprint;
}

Could we write the full fingerprints instead?

[AGWA]

Good idea.

To be clear, git-crypt already uses exclusively full fingerprints internally. The commit message was the only place short IDs were being used.