largefiles: update in two steps, handle interrupted updates better
An update would try to fetch any missing largefiles after having updated normal
files and standins. That could fail or be interrupted and would leave the
working directory in a state where the largefiles not only were missing but
also were scheduled for remove ... and where the old largefile was left in
place.
Instead we now remove old largefiles before starting to download and update
missing largefiles.