lfs: introduce a user level cache for lfs files This is the same mechanism in place for largefiles, and solves several problems working with multiple local repositories. The existing largefiles method is reused in place, because I suspect that there are other functions that can be shared. If we wait a bit to identify more before `hg cp lfutil.py ...`, the history will be easier to trace. The push between repo14 and repo15 in test-lfs.t arguably shouldn't be uploading any files with a local push. Maybe we can revisit that when `hg push` without 'lfs.url' can upload files to the push destination. Then it would be consistent for blobs in a local push to be linked to the local destination's cache. The cache property is added to run-tests.py, the same as the largefiles property, so that test generated files don't pollute the real location. Having files available locally broke a couple existing lfs-test-server tests, so the cache is cleared in a few places to force file download.