Rename t2019 with typo "amiguous" that meant "ambiguous"

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/t/t2019-checkout-amiguous-ref.sh b/t/t2019-checkout-ambiguous-ref.sh
similarity index 100%
rename from t/t2019-checkout-amiguous-ref.sh
rename to t/t2019-checkout-ambiguous-ref.sh