Reduce log output for update command with --restore.
fsck: Emit only one warning per missing blob.
Add a FIXME.
Add remove command.
prune: Add --keep option.
gc: Speed-up and add --amputate option.

gc is now as fast as fsck. The added --amputate option allows the
removal of incomplete trees and broken delta chains instead of just
raising an error. Add information about how much space was freed.