directory CoW on retrieve

directory CoW on retrieve

directory: When cp supports reflinks, use it when getting content from a
directory special remote.

Not yet for imports from directory though, and not for store.

Note that, when it's chunked, using cp --reflink would not speed it up, and
when reflink was not supported, would unnecessarily write the chunk to a
file before reading it back in. So, only using a fileRetriever in the
NoChunks case is necessary to keep chunking fast.

fileCopier is told not to verify, because the special remote interface
does not yet support verification in passing. AFAICS, fileCopies can
never return False when not verifying so the added giveup should never
actually happen.