lfs: only hardlink between the usercache and local store if the blob verifies
This fixes the issue where verify (and other read commands) would propagate
corrupt blobs. I originalled coded this to only hardlink if 'verify=True' for
store.read(), but then good blobs weren't being linked, and this broke a bunch
of tests. (The blob in repo5 that is being corrupted seems to be linked into
repo5 in the loop running dumpflog.py prior to it being corrupted, but only if
verify=False is handled too.) It's probably better to do a one time extra
verification in order to create these files, so that the repo can be copied to a
removable drive.
Adding the same check to store.write() was only for completeness, but also needs
to do a one time extra verification to avoid breaking tests.
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