t7003: check reflog existence only for REFFILES

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7003-filter-branch.sh

@@ -396,7 +396,10 @@ test_expect_success '--prune-empty is able to prune entire branch' '
 	git branch prune-entire B &&
 	git filter-branch -f --prune-empty --index-filter "git update-index --remove A.t B.t" prune-entire &&
 	test_must_fail git rev-parse refs/heads/prune-entire &&
-	test_must_fail git reflog exists refs/heads/prune-entire
+	if test_have_prereq REFFILES
+	then
+		test_must_fail git reflog exists refs/heads/prune-entire
+	fi
 '
 
 test_expect_success '--remap-to-ancestor with filename filters' '