Merge branch 'fs/ssh-signing'

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