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

t/t5703-upload-pack-ref-in-want.sh

@@ -16,7 +16,8 @@ get_actual_commits () {
 	test-tool pkt-line unpack-sideband <out >o.pack &&
 	git index-pack o.pack &&
 	git verify-pack -v o.idx >objs &&
-	grep commit objs | cut -d" " -f1 | sort >actual_commits
+	sed -n -e 's/\([0-9a-f][0-9a-f]*\) commit .*/\1/p' objs >objs.sed &&
+	sort >actual_commits <objs.sed
 }
 
 check_output () {