contrib/rerere-train: avoid useless gpg sign in training

Users may have configured "git merge" to always require GPG
signing the resulting commits. We are not running "git merge" to
re-create merge commits, but merely to replay merge conflicts,
and we will immediately discard the resulting commits; there
is no point in signing them.

Override such configuration that forces useless signing from the
command line with the "--no-gpg-sign" option.

Signed-off-by: Celeste Liu <coelacanthus@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>