t5312: test non-destructive repack

In t5312, we create a state with a broken ref, and then make sure that destructive repacks don't silently ignore the breakage (where a destructive repack is one that might drop objects). But we don't check the behavior of non-destructive repacks at all (i.e., ones where we'd keep unreachable objects). So let's add a test to confirm the current behavior, which is that they are allowed (i.e., ignoring the breakage and considering any objects it points to as unreachable). This may change in the future, but we'd like for the test suite to alert us to that fact. Signed-off-by: Jeff King <peff@peff.net> Reviewed-by: Jonathan Tan <jonathantanmy@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>