largefiles: full debugdirstate functionality for largefiles - just reusing the original command with a mockup repo. This makes it possible to see dates in the lfdirstate and gives less code duplication.

largefiles: make cat on standins do something cat of a standin would silently fail. The use of standins is mostly an implementation detail, but it is already a bit leaking. Being able to see the content of standins might be convenient for debugging.