t6018-rev-list-glob: fix 'empty stdin' test

t6018-rev-list-glob: fix 'empty stdin' test

Prior to d3c6751b18 (tests: make use of the test_must_be_empty
function, 2018-07-27), in the test 'rev-list should succeed with empty
output on empty stdin' in 't6018-rev-list-glob' the empty 'expect'
file served dual purpose: besides specifying the expected output, as
usual, it also served as empty input for 'git rev-list --stdin'.

Then d3c6751b18 came along, and, as part of the conversion to
'test_must_be_empty', removed this empty 'expect' file, not realizing
its secondary purpose.  Redirecting stdin from the now non-existing
file failed the test, but since this test expects failure in the first
place, this issue went unnoticed.

Redirect 'git rev-list's stdin explicitly from /dev/null to provide
empty input.  (Strictly speaking we don't need this redirection,
because the test script's stdin is already redirected from /dev/null
anyway, but I think it's better to be explicit about it.)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>