Test checkout throws if not in repo
Make checkout throw if ref doesn't resolve to hash
Test checkout throws if ref doesn't resolve to hash
checkout() throws if ref is not commit
Test checkout throws if ref is blob or tree
Add todo
Todo done
rm out of date test stub (not going to support this)
Only support diff/checkout for commit objects
Make refs.readHash finally support hash as arg
Move main ref1/ref2 -> diff code inside diff module