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.
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