largefiles: optimize performance when updating (issue3440)
Previously, when updating, cachelfiles was called blindly on all largefiles
in the repository at the revision being updated to, despite the fact that
a list of which largefiles needs to be updated has already been collected. This
optimization constrains the cachelfiles call to only the largefiles that need
to be updated.
On a repository with about 80 largefiles, updating between two revisions that
only change one largefile goes from approximately 6.7 seconds to 3.3 seconds.