Ævar Arnfjörð Bjarmason [off-list ref] writes:

quoted

+test_expect_success 'drop stash reflog updates refs/stash' '
+	git reset --hard &&
+	git rev-parse refs/stash >expect &&
+	echo 9 >file &&
+	git stash &&
+	git stash drop stash@{0} &&
+	git rev-parse refs/stash >actual &&
+	test_cmp expect actual
+'

This one will be portable to the reftable backend.

quoted

+test_expect_success 'drop stash reflog updates refs/stash with rewrite' '

But as I noted in [ref] (but it
was easy to miss) this test will need to depend on REFFILES. So just
changing this line to:

    test_expect_success REFFILES 'drop stash[...]'

quoted

+	git reset --hard &&
+	echo 9 >file &&
+	git stash &&
+	oid="$(git rev-parse stash@{0})" &&
+	git stash drop stash@{1} &&
+	cut -d" " -f1-2 .git/logs/refs/stash >actual &&
+	cat >expect <<-EOF &&
+	$(test_oid zero) $oid
+	EOF
+	test_cmp expect actual
+'

Why should this be tested with "cut" in the first place, though?

If we start from

    stash@{0} = A
    stash@{1} = B
    stash@{2} = C

and after saying "drop stash@{1}", what we need to check is that

    stash@{0} = A
    stash@{1} = C

now holds, which can be done with "git rev-parse", and the fact that
the ref-files backend happens to record both before-and-after object
IDs is an irrelevant implementation detail, no?

I am still puzzled.