Merge branch 'fs/ssh-signing-key-lifetime'

Merge branch 'fs/ssh-signing-key-lifetime'

Extend the signing of objects with SSH keys and learn to pay
attention to the key validity time range when verifying.

* fs/ssh-signing-key-lifetime:
  ssh signing: verify ssh-keygen in test prereq
  ssh signing: make fmt-merge-msg consider key lifetime
  ssh signing: make verify-tag consider key lifetime
  ssh signing: make git log verify key lifetime
  ssh signing: make verify-commit consider key lifetime
  ssh signing: add key lifetime test prereqs
  ssh signing: use sigc struct to pass payload
  t/fmt-merge-msg: make gpgssh tests more specific
  t/fmt-merge-msg: do not redirect stderr