t2019: don't create unused files

t2019: don't create unused files

Tests in t2019-checkout-ambiguous-ref.sh redirect two invocations of
"git checkout" to files "stdout" and "stderr".  Several assertions are
made using file "stderr".  File "stdout", however, is unused.

Don't redirect standard output of "git checkout" to file "stdout" in
t2019-checkout-ambiguous-ref.sh to avoid creating unnecessary files.

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>