Max Gautier [off-list ref] writes:
$ git rev-list @{u}..HEAD --format='%G? %H'
commit 9497d347b048dbea7f527624f815f7926594c4bc
error: gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification
"git rev-list" as a plumbing does not want to read in or be affected
by unnecessary configuration variables, so it uses
git_default_config() to read the barebones, unlike "git log" that
uses git_log_config() that calls, among other things,
git_gpg_config().
Untill "git rev-list" learns to inspect the --format string and
checks necessary configuration on-demand (e.g. presence of "%G"
signals the need to call git_gpg_config()), this is sort of
expected, unfortunately.