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

diff --cc builtin/receive-pack.c
Simple merge

diff --cc fmt-merge-msg.c
Simple merge

diff --cc log-tree.c
Simple merge

diff --cc pretty.c
Simple merge

diff --cc send-pack.c
Simple merge

diff --cc t/t4202-log.sh
Simple merge