Git integration

[chromakode]

It should be possible to create attestations about git commits. Since a git commit is a sha1 hash of repository content, it should be a suitable unique identifier within signet.

When sig attest is given a repo directory (+ git commit?), it should use "git:" as the id property.

This should probably be implemented as a plugin, since it's not necessary for everyone's uses, and adds complexity to the core.

• Identify git repos with git:<commit id> identifiers
• Create attestations about git commits
• Attribute git remote origin URL in attestations?
• Check if an earlier commit in a repo has attestations and show diff