upload-pack tests: avoid a non-zero "grep" exit status

upload-pack tests: avoid a non-zero "grep" exit status

Continue changing a test that 763b47bafa (t5703: stop losing return
codes of git commands, 2019-11-27) already refactored.

This was originally added as part of a series to add support for
running under bash's "set -o pipefail", under that mode this test will
fail because sometimes there's no commits in the "objs" output.

It's easier to fix that than exempt these tests under a hypothetical
"set -o pipefail" test mode. It looks like we probably won't have
that, but once we've dug this code up let's refactor it[2] so we don't
hide a potential pipe failure.

1. https://lore.kernel.org/git/xmqqzh18o8o6.fsf@gitster.c.googlers.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>