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

Documentation/config/gpg.txt

@@ -64,6 +64,11 @@ A repository that only allows signed commits can store the file
 in the repository itself using a path relative to the top-level of the working tree.
 This way only committers with an already valid key can add or change keys in the keyring.
 +
+Since OpensSSH 8.8 this file allows specifying a key lifetime using valid-after &
+valid-before options. Git will mark signatures as valid if the signing key was
+valid at the time of the signatures creation. This allows users to change a
+signing key without invalidating all previously made signatures.
++
 Using a SSH CA key with the cert-authority option
 (see ssh-keygen(1) "CERTIFICATES") is also valid.