t/gpg: simplify test for unknown key
To test for a key that is completely unknown to the keyring we need one
to sign the commit with. This was done by generating a new key and not
add it into the keyring. To avoid the key generation overhead and
problems where GPG did hang in CI during it, switch GNUPGHOME to the
empty $GNUPGHOME_NOT_USED instead, therefore making all used keys unknown
for this single `verify-commit` call.
Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>