Removes a check-in and the blobs only it introduced, into a local graveyard that purge undo restores and purge obliterate empties.

The drop set is (seed + reachable(seed)) minus reachable(everything retained), so a blob a surviving check-in still names is never freed -- dropping it would create a phantom rather than reclaim anything. Only leaves go, judged over the whole named set: purging a parent alongside its child is accepted, purging it alone is refused.

Reuses shun's rewriteAndSwap and records no tombstone: a purge is local, so content that already synced comes back from a peer. The help says so and points at shun. check-in: 8536f59fba user: dmestas tags: trunk