Ability to clone repos with the last commit before the deadline

Recently classrooms updated too support a _hard deadline_ so assignments can move to read-only post deadline.

If you don't want to enforce this, but still want a snapshot of the state of all repos _just before_ the dealine, you must introspect the commits once cloned and checkout appropriately to that commit.

I want an option to checkout a single commit (#15) that is the last before the deadline, or another option to default to the latest commit.