lfs: add the ability to disable the usercache While the usercache is important for real world uses, I've been tripped up more than a couple of times by it in tests- thinking a file was being downloaded, but it was simply linked from the local cache. The syntax for setting it is the same as for setting a null remote endpoint, and like that endpoint, is left undocumented. This may or may not be a useful feature in the real world (I'd expect any sane filesystem to support hardlinks at this point).