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.