largefiles: don't cache largefiles for pulled heads by default
After discussion, we've agreed that largefiles for newly pulled heads should
not be cached by default. The use case for this is using largefiles repos
with multiple remote servers (and therefore multiple remote largefiles caches),
where users will be pulling from non-default locations on a regular basis. We
think this use case will be significantly less common than the use case where
all largefiles are stored on the same central server, so the default should be
no caching.
The old behavior can be obtained by passing the --cache-largefiles flag to
pull.