With SIGNING_FORMAT = ssh the OpenPGP verification path builds its
GPGSettings from the instance signing key but leaves the format empty,
so it runs `gpg -a --export` on an SSH public key path. Depending on the
local gpg setup that either exports nothing, so an OpenPGP signed commit
reports gpg.error.generate_hash instead of a missing key, or it fails
outright and logs an export error for every such commit.
Both guards are needed. The first covers SIGNING_KEY set to a path with
SIGNING_FORMAT=ssh; the second covers the shipped default
SIGNING_KEY=default, where the format comes from git's own gpg.format
and never gets reconciled with the hardcoded "openpgp". Drop either one
and a working config goes back to broken.
Also raise minimum git version to 2.18 which was already required before this change.
Fixes: https://github.com/go-gitea/gitea/issues/37452
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Artem Lytkin