largefiles: import whole modules instead of importing parts of them

Be more friendly to demandimport.
largefiles: fix profile of unused largefilesdirstate._ignore
largefiles: remove confusing handling of .bad return value - it is void
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.
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: use more reasonable locking for update
largefiles: clarify installmatchfn documentation
largefiles: copy override, install matchfn outside the try/except restoring it
largefiles: remove silent handling of incorrect invocation of restorematchfn

It is better to get a crash than to continue without noticing errors.
largefiles: remove dummy assignments for creating an already existing closure
largefiles: simplify revert - use getstandinsstate like other commands do
largefiles: revert override, install matchfn outside the try/except restoring it