t6050: redirect expected error output to a file

t6050: redirect expected error output to a file

Otherwise the error from `git rev-parse` is uselessly
polluting the debug output.

Redirecting to a file, instead of /dev/null, makes it
possible to check that we got the error we expected, so
let's check that too.

Reviewed-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>