Pushing an empty pack should be fine. There was an earlier issue where
deleting refs didn't work because we tried looking for objects in the
uploaded pack and didn't find any. So we do want to preserve that
functionality. We also should be able to accept new refs that point to
commits that are already on the server.

missing objects are actually bad, so we don't need to have pushes work
when a partial graph is supplied. For now, the push still appears to
succeed, but in the next step it won't.

We have this enabled in our prod config, and it changes
spokes-receive-pack in important ways. In particular, it makes
spokes-receive-pack care about bad dates, which we want to disable
sometimes. So let's enable it here (and update the existing tests to
match the new behavior.

… requested

In general, it's better if customers rewrite their repos to remove the
bad dates because bad dates aren't guaranteed to work with all of our
systems and APIs. But, sometimes there are bad dates in a corner of a
repo where they're unlikely to cause problems, and the cost of rewriting
the repo is too high to be worth it.