Fixes#35884
Problem: Since `once_buf_loaded` might call a callback passed to it at a
later point (which is it's reason to exist) that callback might end up
referring to stale diagnostics in a handler's `show` function. For
example, if we first call `vim.diagnostic.set` for an unloaded buffer,
then call `vim.diagnostic.reset` and only then load the buffer, we might
still see diagnostics from `vim.diagnostic.set` call, which are stale at
this point.
Solution: only keep one autocommand from the most reset `show` call and
delete it when `hide` is called.
a1bc5d0ae6