import: (v2) delete writes the blob into history in subdir

This makes it easier to audit deletes with "git log -p" and
prevents an unstable specification of "content_id" from being
stored in history.

This should be cost-free if done in the same partition (and even
cheaper than before as it introduces no new blobs).  It does
have a higher cost across partitions, but is probably irrelevant
given the typical ham:spam ratio.