`ipfs add --nocopy` shouldn't send file data to go-ipfs

`ipfs add --nocopy` currently acts exactly like `ipfs add` except that filestore references are written instead of blocks. However, we're still _sending_ the file content over the HTTP API (before throwing it away).

Ideally, we wouldn't have to send the data in the first place. However, fixing this in go-ipfs-cmds will be non-trivial (and may not be worth it in the grand scheme of things).