lfs: override walk() in lfsvfs In order to fix the missing lfs store after an upgrade, I attempted to walk the store vfs to hardlink to the upgraded repo's store. But the custom join() clashes with the default walk() implementation. First, 'path=None' blew up in the regex matcher, because it wanted a string. But even if that is fixed, the join to walk the root of the vfs wouldn't match the required xx/xx...xx pattern. The first cut of this was a copy/paste/tweak of the base implementation, but this version of walk() hides the internal directories, and treats the vfs as a flat store. I think this makes sense because most vfs methods call join() on input paths, which wants the simple oid format. It also relieves the caller from having to deal with bogus files/directories in the store.