Merge branch 'jc/push-cert' into maint

The "git push --signed" protocol extension did not limit what the "nonce" that is a server-chosen string can contain or how long it can be, which was unnecessarily lax. Limit both the length and the alphabet to a reasonably small space that can still have enough entropy. * jc/push-cert: push --signed: tighten what the receiving end can ask to sign