Merge branch 'fs/ssh-signing'

Use ssh public crypto for object and push-cert signing. * fs/ssh-signing: ssh signing: test that gpg fails for unknown keys ssh signing: tests for logs, tags & push certs ssh signing: duplicate t7510 tests for commits ssh signing: verify signatures using ssh-keygen ssh signing: provide a textual signing_key_id ssh signing: retrieve a default key from ssh-agent ssh signing: add ssh key format and signing code ssh signing: add test prereqs ssh signing: preliminary refactoring and clean-up