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.